Underfeeding him - just let him feed from one breast at a time, don't offer both sides. If he's still hungry after his burp a few minutes later then feed (sometimes he's just peckish so doesn't request more after this, but he woudl have had it if I'd have offered, but he feeds very regularly during the day as he's never fully tanked up except for at bedtime) - if I give him both sides in quick succession he will bring his feed back.