Hey Rachael! OTTBS are almost exclusively ridden in a very basic O or D ring snaffle bit on the track so that is what I'd use with him as that is what he is accustomed to wearing.
Most Ottbs are very short on ground manners since time is money that part of their training is often lacking. Sounds like your guy could be the exception (but I wouldn't be on it LOL) but that is the place I'd start. There is a wonderful
book written buy one of the founders of New Vocations that completely will answer all of your questions about retraining an OTTB.
I've retrained many OTTBs and I found it to be right on target. It also gave me many good tips.
I think the name is Beyond the Track
A huge percentage of OTTBS have ulcers so I'd do yourself a favor and get your baby in some ulcer meds right out of the gate. I use Smart packs ultra digest. Especially look into this if you find he's losing weight on his normal rations. OTTBS will eat 4 times what other horses you may have had eat also :)