I'm a big believer in vaccines. If you don't get rabies vaccine for your horse and he gets rabies from a rabid groundhog, it is a horrible situation. Your horse will die a miserable death. All of the people who were around the horse will need thousands of dollats of painful, rabies immune globulin and rabies vaccine. All of this could have been prevented with a $10 rabies vaccine.
I think that we way over vaccinate our horses but foals are a different story. They have antibodies from their dam that last approximately 6 months then they need to start making their own. That said, I don't think it causes much of a risk if any to the horses around him if he isn't vaccinated. If he gets west nile or tetanus for example he may die but won't infect others. To bad he has a jerk for an owner, the future doesn't sound real bright for him.