Hi Jen! I have a magic trick that may help you!

(Hopefully...)
This always worked for my daughter when she was really little:
I would give her a bottle, stop halfway, and then lay her down to change her diaper. It usually took me a little longer to change her diaper in the middle of the night because I was soooo tired, lol. When I picked her back up to feed her, I always got a big burp because she had time to "work one up." I think laying her down really gets it to build up. Then when I picked her up, it usually just came on its own. So, maybe you can try to lay your son down on your lap for a bit while you wait for the burp.
Do you burp after every few ounces or after an entire bottle? I know that at four ounces my daughter went through a huge growth spurt and could take in more at a time before needing to burp, so she required more formula, but less burping. Maybe it's possible that your son needs more ounces intitially before getting a burp. Just a thought.
Good luck to you!