If he's allergic, have you tried an elimination diet? If you can pinpoint what he's allergic to, you can maybe improve not only his tummy, but his skin and fur.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."