D.L.
Our little girl has always done that right before getting sick. She still has nights like that now at 2 1/2 when she's not feeling well. I also agree that it could be teething.
Another thing is that he might be going through a growth spurt and/or is hungry more than normal. He could also be ready for a developmental jump. Our little girl has always had a hard time sleeping right before a big breakthrough (standing, walking, etc.).
I would call his doctor about his belly feeling hard, however. I know that our pediatrician has said that a hard tummy is not a good sign and that tummies should be soft. It's probably nothing, but better to be safe. Hope you get more sleep soon! :-)