as I read I thought maybe he was eating too fast.. [I´ve had a few dogs that would cry right after eating.. then throw back a lil food.. so I had to make sure they ate slowly..

the thing that throws me off is the bum licking.. it could be his way of showing anxiousness.. or maybe an allergy as suggested.. one other thing that came to mind was if he was dewormed?..

you can try making him eat very slowly to rule that out.. you can deworm him..and you can switch to a less allergic food..

hope you find what´s making him uncomfortable..