A few days ago, while working out for the AD, my dog became lame. By the next day, he wouldn't put any weight on the foot at all. There was no open wound. If I compare feet, I can see that the big pad is swollen at the top point and if I feel it, it feels like it has fluid under it. I'm assuming this is a blister.? I gave him some aspirin for 2 days and left him totally off. The third day he barely had a limp so we did some ob in the house. Yesterday was the 4th day, no limp at all, no noticable swelling. Went out on the bike and we ended up only going a mile because I could see the limp coming back. Once we got back and he was on soft footing, he was fine but very ouchy on rocks. Checked the paw again. Fluid filled pocket. I don't know if I missed it before or if it came back in this short time...Im inclined to think I must have missed it because the swelling was down.
Now....what to do? Continue to let him be off work until it's all the way gone? Drain the fluid?...I'd think that would leave him open to infection.
The vet isn't an option until it's the only option because since he was neutered, he is insane at the vet's office.

I would definitely keep him off it.

I know you don't want to bring him to the vet if you don't have to but this might be a case of "have to". My first thought is that maybe there is something (ie: needle, twig, something else sharp) that became stuck in the pad, migrated up and is now forming an abscess.

If this were me, I would get the dog off it's feet for a few days (which you have already done) and then if it persisted (which it is) I would be inclined to bring my dog to the vet. It may be nothing, but it may be something. And if it is an infection, I would not want to take any chances with it getting worse.