My puppy, Roxy, just got her Lepto shot, and she's going to get her booster for it tomorrow. I didn't see any reaction with it in her, but my vet did tell me to keep an eye on her. The way my vet made it sound is all they have to do is give them a shot to reverse the affects if she does have a reaction. She said that it was common in small dogs to get reactions.
My mom's dog has severe reactions to shots too, he gets seizures from some certain shots, its really scary. She has decided not to give him his shots unless the virus becomes very common in our area and it is something to worry about.
I would ask the vet if it's something to be concerned about if he doesn't receive the Lepto. If they say you should, maybe you can leave him there and they can give him something to make the reaction go away when it becomes present. If they say they are not really worried, I wouldn't do it.
Good luck to you guys!