XXXXXXXXX,
It sounds like he is consistently painful on his abdomen so I think it is worth a trip to the vet. It's hard to tell in such big dogs if there is a mass in their belly or not because they often resent pressing on it. It could be just an upset stomach, or it could be an inflamed pancreas or a mass. But a simple bloodtest, good examination and perhaps some xrays will put your mind at ease.
When you go to see the vet try to narrow down the time the signs started and if its getting worse or better. That will help the vet out.
Good luck and I hope everything goes well.
Dog Veterinarian
4+ years additional specialty training after vet school