I think the cause of his bloated stomach has to be discovered first, and treated. He will need to see the vet.
There are a number of things which can cause bloated stomach....liver problems, heart problems, pancreatitis, internal bleeding....and other things. He needs to be checked out properly.
I the cause is found, then if any treatment is started, it should help him regain his appetite naturally
I hope there is some improvement soon.