Twitching isn't a real good option during hock injections.  
I don't see how your horse could be allergic to all tranquilizers.  For example, allergic reactions to Acepromazine are very rare.
There are a lot of different tranquilizers available out there, I'm sure there's one he won't react to.
There are also several other classes of drugs that can be used, that cause them to hold still, but aren't tranquilizers, like Rompun.
What tranquilizer was he given and how did he react to it?  Hives, throat/breathing problems?
If the horse is having periodic hock injections, that sounds like he has some arthritis ('spavin') in his hocks.  He could get a vacation from work for a while, til he gets his injection, or he could be given an anti-inflammatory medication; your vet is the best person to answer those questions as he knows the horse's history.
By the way, guys, if your horses are having any hives due to environmental allergies, you might ask your vet about using Claritin.  Some people round here are trying it and having good luck.  It's inexpensive and seems to be effective, but do please discuss it with your veterinarian - don't give your horse any meds, supplements or other treatments before discussing it with your vet....