when they are little thier tummies arent the best at digesting and formula is alot harder to digest than BM anyways so thats probably why, are you giving him water betwen feeds too?
if it continues talk to your GP, DD has bad constipation (thinking she has irritable bowel like XP) so she is on a constipation formula and coloxyl drops but TBh its probably just cause hes little but if in doubt see a GP HTH