Our recently adopted 10 month old Dachshund jumps up and snaps at my 19 month old son. What can I do?

Hello,

I have a concern about our 10 month old dachshund we adopted from a dachshund rescue web-site in Nebraska. He is a intact male, but just recently had him neutered. We have only had him for 3 weeks now, we love him and enjoy him very much, he is so far very good with our oldest child, my husband, myself and our other little dachshund. He doesn't get along well with our 19 month old son. I think he is a fear bitter, and I really have to defend the dog on this part because my son is so naughty and tends to bother him a lot. We do discipline our son and do try to keep them separated, but sometimes being in the same household it gets hard to all of the time. Anyway, he tends to snap or bite at our youngest son even when he enters the room, especially if he is on my lap at the time, he starts growling then gets off my lap and jumps up and bites him. Even if he isn't doing anything at all. He also does this to our Female 7 year old German Shepherd. She does stay outside most of the time anyway, so we do keep them separate and she never has bitten back yet, she will just leave him alone or walk away when he does this. But I am worried about my younger son. I haven't disciplined the dog yet, because I just assume he was maybe abused or is just scared of him. Anyway, I would like some advise as to where to start or just wait and give some time with the dog??? I am mostly just concerned about our youngest son. He doesn't mind our 4 year old at all, he will even sit on his lab without any problems, it is just our 19 month old and our German Shepherd. I would love it if someone could give my any suggestions on what to do, if anything at all at this point. Thanks so much for your time!

Heidi