My newborn is constipated what do I do.??!

Crystal
My 3 week old son hasn't pooped all day.! Since like yesterday mid day. He's been in a bad mood all day n I see him pushing but all he does is pass gas. I think its because I breastfed him for 2 weeks straight n recently like 2 days ago I started giving him infamil formula because I feel I don't fill him up with my milk at all so now I kinda do both but slowly want to just switch him to Similac formula. I'm worried I don't know what to do