If bananas were not known for being constipating, I would suggest that.
Avacado is considered a good first food. And you can buy one and just mash it up yourself. He's still pretty young though, so holding off on solids for a few days and just nursing would probably work pretty well too. As BM is a good laxative.
You could also try rubbing his tummy in clockwise circles, this is suppose to help. Just don't do it in the other direction, as that can cause the opposite problem. You can also try bicycling his legs, that helps some too.