Trouble with burping.

Hey mommas and daddys! I am writing to this post because I've had my newborn home for a few days now and I'm having quite a hard time trying to get him to burp after feedings. Now, mind you- he was born at 36 weeks and is only 4 lb, 15 oz right now so he doesn't take in a whole lot at a time as is. He drinks about 45 ml at a time every few hours and he has yet to show any signs of difficulty eating or gas pains. Sometimes he will burp after his feedings, but mostly he won't. I have done a small amount of research and saw that it could be because he's still getting such a small amount at a time, so he's not ingesting much air. Has anyone else had this problem; especially with a preemie? Like I said, he has not shown any signs of distress whatsoever and of course I try to burp him after every feeling, but I just want to make sure I'm doing this right. (First time momma). Oh, and he's formula fed. Anyways, any input, advice, or positive vibes would be appreciated! Thanks :)