Constipation????

I have a 6 week old. He is fed 4oz (2 of breastmilk and 2 of formula-- Similac Pro Sensitive) every 2-3hrs. He is cluster feeding right now too.

He usually poops at least every other day or daily. This is day 2 and no poop still. He seems to be straining a little tho but just gas comes out.

We have tried belly massage and bicycle legs. Any other suggestions?