also if he was friends with a new bloke this bloke could be in better shape than him.
The working out thing could even be insecurity that he isn't what he used to be and is just as worried about you cheating as him!
I hope it's something as innocent, I really do.
FWIW, my ex cheated, loads, it was a totally different reaction if I questioned his whereabouts, plus I think you can tell if your dp/h is hiding something, not necessarily what, but normally you know the truth when you see it. DP was so embarrassed about telling me what his mates had said.