He may be constipated because his gut isn't working. My mother used to put karo syrup in the formula for all babies of whatever species to keep the bowels moving along. Glycerine infant suppositories work well too. I have no idea why your vet is suggesting an antihistamine, but benadryl will work.
Vets who are not familiar with goats may not recognize polio, Polio causes some truly bizarre symptoms in goats and if not treated promptly and aggressively it can be deadly.
I am assuming your goat has or had entero. I am also assuming that because of the entero it now has polio. Enterotoxemia is caused by a digestive upset that allows the causative organism to proliferate. The clostridia produce a toxin. Thiamine is produced in the digestive tract. For that reason, whatever caused the entero caused a suppression in the production of thiamine, hence the polio. For entero you need to give clostridium perfingens antitoxin. Sometimes more than once. A course of penicillin is helpful because it is effective against the clostridial bacteria that cause entero. To treat entero, we first gave a shot of clostridium perfingens antitoxin followed by a dose of thiamine and then some banamine if she was in pain. Then we put the goat on a course of penicillin for five days. It sounds like your vet did all that except for the thiamine. If your goat is still acting like he is blind, he needs thiamine ASAP.
By the way, almost all shots for goats should be given subcutaneously, not in the muscle. That isn't just my opinion. That is what the vets at the vet school at UC Davis told me to do.
This post is a little long. I hope i didn't tell you more than you wanted to know. Good luck with your baby.