'emmeline', I am so so sorry that this has happened ! The joys of dog parks !
Hindsight is a wonderful thing - but - really - you have done the right thing - not like me years ago - when I went in and tried to stop a dog attacking my pup and not only did I get put on my butt very quickly - but more damage was done to my pup.
With your pup cowering and such - please remember he will be in a lot of pain and not feel like doing anything for a while - and with all the emotional stuff going on around him - he probably is wondering what he has done wrong. So gentle, gentle and slow !
If you have ever had any trauma to your ribs - you will know that just even breathing is painful. He will eat when he wants to. I would be more inclined to make sure that he has plenty of fluids - more than food at this stage - and make sure that he is comfortable.
You can be as ingenious as you like with tempting him with food - only you know what he really likes ! With my pup, I was mashing up bananas, dry kibble and sardines with water and making ice blocks out of them. Also putting ice blocks in his drinking water to get him to drink water.
Take care - the both of you !