It sounds like the Zantac isn't enough. Prevacid comes in a solutab that you add to applesauce or similar. You can take it instead of or with the Zantac. I had one who did well with Prevacid, one who takes both Zantac and Prevacid. If he's had reflux all his life, and has had difficulty swallowing, he probably needs some feeding therapy to learn to use his mouth properly. We had to do that with our two reflux kids. If he has apraxia, nothing but therapy is going to help. We had/have 3 kids whose apraxia affected their speech, two of whom also needed feeding therapy. Child Find will evaluate all of that for free, and then come to your house to do the therapy.
If your doctor isn't willing to change medications, or at least up the Zantac, you need to get a second opinion. When reflux medicine is doing it's job, there is no choking (assuming correct muscles are being used), and no dry cough. It sounds to me like your son has issues with thin liquids because it has always hurt to drink. If you can't get services in your area, try Simply Thick. You just mix it in with whatever his drink is. You can start out with nectar thickness, and thin it as he does better. You have to order on line, but it's so much better than anything else on the market! The oatmeal thing definitely is a texture thing. It's difficult to swallow. Anyway, I know I was rambling. I hope something in here helps. You didn't mention it, but if he has weight issues, I can give you a yummy recipe for a formula substitution using the Simply Thick. GL!