Quite possibly oversupply with overactive let down which is filling his tummy up to fast.
It could be a bit of a tummy bug or possibly even food intolerance, so you can keep an eye on it, but I would not be too concerned unless there are worsening and/or other symptoms going on.
Mommy to DS born 11-10-10 And DD born 6-3-13
DD would do that if she ate too much. I started limiting the amount that she could eat when she was fussy/seemed like she had a tummy ache. It resulted in her eating more often.... but much less throwup and she's easier to burp. I also try to wait 20 minutes after puke to feed again to let her stomach settle. Sometimes she won't let me but I at least try not to nurse her again right away.