Baby not feeding well

Hiya, my little boy who is 3 weeks old has stopped eating properly he won't do more than 5 minutes on the boob or even take formula (combi feeding) he will go on to the boob and latch but then fall asleep. He's been getting really bad trapped wind which he is straining to get a poo/trump out of him which is then causing him to be sick through nose and mouth. I feel so helpless that I don't know what to do to make him feel better. Any one else had this? Any tips?