Baby constipated. What to do??

My 2.5 month old baby hasn’t pooped in 2 days. I’m starting to get concerned. He’s been a little fussy the last few days and not eating his bottles as well. He will eat 2 ounces and then be full, but will drink the other 2 ounces 30 mins later. I’ve tried rubbing his belly, and moving his legs to try and get things moving. But nothing has happened... suggestions???