I agree with Melanie - my b/f bub only goes every 5-6 days and that's normal. But banana is defnintely constipating so I'd ditch that and go with pears at first.
As someone who had a really constipated formula fed baby with my first bub I can tell you that if he's constipated you WILL know about it. They are not happy little campers and there will be much whining and writhing and grunting. Two days is nothing, and if he's happy and not bothered I'm sure he's fine and poopey will probably make an appearance in the next day or so if that's his normal "habit"