2 week old constipation. HELP!

Krishna
I was given antibiotics, Tylenol and ibuprofen for an infection. I have been exclusively breastfeeding since day 1 and right and since Wednesday my milk supply decreased dramatically so I had to feed him formula. 
He hasn't pooped for almost 2 days and today he has been in pain. He pushes and NOTHING. He is so frustrated. 
Any safe ideas that you mamas can share with me?