I agree that you should consider the possibility that your son might not be allergic to garlic since he was diagnosed through testing only, and you're wise to seek an allergist to do a food challenge. Finding one who's an expert on garlic allergy will likely be near impossible since it's not a very common allergy. I'd suggest just finding an allergist who is very familiar with food allergies and up on current research, testing, diagnosis, treatment, etc. If you're comfortable sharing the region where you live, I'm sure our members would have suggestions of which allergists are good in your area.
As for how to introduce the garlic, I'm not really sure. The allergist doing the challenge might have some specific protocol (s)he follows. If not, I'd probably want to use a food that I know my child likes that tastes good with garlic added. Like bread w/ garlic butter instead of regular butter.