If he starts to get wriggly or pull off the boob it could be air...
Just pat his back or jiggle him gently until he brings it up - sometimes he might bring some milk up with it and thats ok too.

Just remember your both still learning to feed - if you did want to drop the formula now would be a good time to do so (it's ok if you don't want to) as it takes about 6 weeks for your supply to settle into a routine.

Even now at nearly 5months DS will feed and then 10 mins later let out a huge burp or fart and then start whinging for the boob again