OMG poor poor Tommy
I can't offer any advise ... thankfully never been through anything like this. But I agree the RSPCA should be informed ... it might also be worth informing the local police. If someone is taking pop shots at cats they need to be made aware, it might be a person next time.
Please pass on my best wishes to Tommy's owner and I will be keeping everything crossed that he gets well soon.
BTW I would suggest his owner tries him on "smelly" fod ... sardines in tomoato sauce are usually a good way to get a kitty eating again. As he is jumpy and scared it might be worth leaving a radio on quietly in the room that he is in, this will stop him being starled by any noises and maybe a felliway plug in to help calm him down.