Medicine past Gag Reflex

Try giving him a reward for getting it down. Offer him a piece of candy or some small prize. It's gonna be hard getting it down, but if you can offer him an incentive, maybe he'd be more willing? Not sure how old your child is, but just thought it might be an option. My son has the same issue, so I feel your pain.

Not sure why stirring it into hot chocolate or a bite of potato or something is out (That's how I taught myself to swallow pills, actually, by "chewing" first), but....

Chloroseptic spray. Will make his sore throat feel better and also numbs the gag reflex.

Have him put the pill high on his tongue and have him swallow it with orange juice. If it is a capsule break it open and mix it with honey so he can swallow it.
The gag reflex will get less difficult when his mouth changes shape around the age of 10 years.

Often it's the tongue that sparks the gag reflex. If the antibiotic is a pill, have him put it in his cheek, like a chipmunk with an acorn. Don't let it touch his tongue. Then have him drink water or milk or whatever from a sports bottle. Using the sports bottle with those sipper tops makes you tip your head back further. The pill should go down. (If it's a liquid, squirt it with the syringe into his cheek).

I would ask the doctor to give your son the liquid antibiotic if you are now giving him a pill. If you are already giving him the liquid and that is what he is gaging on, and it really is becoming an issue of him not getting the medication, then you can speak to your doctor and he can be given one shot in the office. My son has an issue with medication. He will only take motrin and only a specific flavor. When my son has strep or an ear infection and he must get antibiotics the doctor knows my issue and he gets an injection in the office. We have done this 3 times now, and it is pretty great because they generaly are all better by the next day.
In trying to teach my older daughter how to swallow pill form medication my doctor suggested that you take the mini M&M's and try swallowing one whole with water just like a pill. When you can do that with out a problem you move on to regular size M&M's which are about the same size as a regular tylenol. She said that kids are less freaked out because they know that M&M's are good.

I don't know if it's possible, but do antibiotics come in suppository form? He may not appreciate that any more than gagging, but it might ensure he gets the medication he needs...