If possible get him to open his mouth and see if you can see any signs of infection. The only other thing I can think of is very early stage MBD. Are you using a phospherus free calcium supplement containing vitamin D3? If you already are, and can't find any signs of infection you might want to get a good UV light and see if that helps. Most UV lights have a six month lifespan so if you have an old one it may need to be replaced. I've raised als without UV, but it's kind of like walking a tight rope in terms of avoiding MBD. I now use UV for piece of mind and I think that it does make for more more robust als.
-Alice