You may want to clean up the house of items your current dog may feel protective of like Connie mentioned toys, also bedding or food bowls.
Your dog has had you and the house all to himself and may resent having to share the attention with a new puppy being brought into his house.
You may want to have the first meeting on neutral ground like a park or somewhere away from the house, you bring the puppy and your wife could bring the Lab separately to see how the initial meeting goes.

We had already planned on introducing them on neutral ground. Keeping toys away when they're together shouldn't be a problem. I was wondering if we should try to keep them separate for a while until the puppy settles in and if so, for how long.

If you want to separate them a little in the begining, don't do it by changing the Labs routine. I think there's an article here about introducing a new dog and a little bit of a time table that may help give you an idea, but I think for the most part it isnt too tough because when you bring home a puppy, you're limiting their access to a lot of things, crate training, etc..

Even if they seem to be getting along I wouldn't leave them together alone for a minute(without supervision) for the first few weeks maybe months.

I may be overly paranoid because I made that mistake of going a bit to quick and it spelled disaster, so now for me I like the idea of taking introductions slow, no rush.

It's gonna be different with every dog. I have 2 female Rotties we got 1 (Tibby) and 3 months later the other (Fritzi). But at 3 months Tibby was already conditioned to NO and I already had an really good bond/pack leader relationship with her. When we brought Fritzi home Tibby was more not curious than she was curious. Dogs cannot reason like people so they will not thik to themselves "oh no what are you doing with THAT thing in our house" or "OH NO now I'm gonna have to learn to share" ... as mch as some people want to think their dog is like a human, it's not ... it's a dog.
With that said all we did was took Tibby outside on her leash, Fritzi was in her crate closed up. First I let Tibby see inside the crate, let her smell etc until she acted like she sort of wasn't interested. Then I had my daughter open the crate door and let Fritzi come out on her own. At first Tibby was more timid about the new little creature than she was agressive, in fact she wasn't agressive at all, not really even playful lol. She acted more like she just didn't want to have anything to do with "it".
I only keep my dogs in crates until they have bowel control, at around 14 - 16 weeks and a thousand times of taking them in/out of the house they are on their way to being house trained. I'm lucky enough to be retired & home 24/7 with them.
Anyway each day when we let Fritzi come out of her crate after a nap, I would just sit o the floor with Tibby and hold her leash so she couldn't "charge" the little puppy. I would pet her and talk to her to keep her calm while Fritzi played on the floor with my kids, when Fritzi would start coming close I would just keep Tibbys attention on me as much as I cold, but still let her notice & interact. I would have Tibby on one side of me and get Fritzi to approach me from the other side. It only took a few days until Tibby figured out that little thing wasn't a threat to her. After about 10 days they were fine. I still kept an eye on them when they played and if Tibby started to play to rough or just get to happy I would just redirect her with a calm NO and they did fine. Now they are adults and they get along great, I make sure if 1 dog gets a treat so does the other and it's the same kind of treat. We play tug both together and individually, they each have their own space but many times they will lay right next to each other. At night one sleeps on my wifes side of the bed the other on mine, and they dont care who gets what side as long as they are both in the room with me. I am the pack leader, I have never tried to 'dominate' them, I am just a fair but firm pack leader.
I grew up helping an Uncle train labs, they are for the most part calm submissive dogs, and I would think that as long as you make sure to share your time & attention equally, and even dedicate more to the Lab in the first few monts, this new puppy should not be a problem introducing & having around. You may end up with 2 dogs that are best friends, & don't mind sharing their toys & humans.
I would agree with certain breeds & less experience handling them, it could certainly be a bad situation. The main thing is make sure you have a good bond & can verbally control/calm the lab. good luck & take videos to put on youtube

Thanks for the replys everyone. I think I have enough information to make this work.

Connie, I'm curious wether you think I'm right about the labs submissiveness towards other dogs or not. Wouldn't you think it's due to the aggression from the boxer as a puppy? She acts perfectly normal with people.

Kory

Not Connie but aggression from an older dog against a pup will often create a dog aggressive dog if it has any effect at all.
Out of fear the attacked pup can grow up thinking it has to make the first move in order to keep from being attacked. Again, that's a fear based aggression.
Being submissive to other dogs is a behavior that is usually genetic.
Pups in a litter determine their status with one another. The submissive pup will never truly be top dog and the top dog will never truly be submissive.
Even with that, a particular pup may be top dog in it's particular litter but low on the social ladder with dogs outside the litter. It still boils down to the character of the individual dog.

Interesting. When my wife and I met, she had a dog that had been beaten by her X a couple of times pretty severely. One of the many reasons she left him obviously. Anyway, that dog was extremely submissive when approaching men for many years after that. I'd say it was probably two years or so before he stopped bowing his head even when approaching me who he'd been around every day. That's why I assumed this dog's being submissive to other dogs was due to the aggression from the boxer.

Like Bob is saying Kory, submission and fear are two different things. Submissive dogs are comfortable with being submissive. The abused dog going into avoidance like that isnt the same. I would look at how it will effect your puppy in that you don't want him to bully the lab because that submission can make her a target for some dogs. They'll just naturally want to bully her and thats not something I want my dogs to do.

O.K. so I was confusing avoidance with being submissive. That does make sense.