I don't think he's slipping in his actual martial arts experience, although I haven't seen anything recent. I think his girth
doesn't allow him to do anything that looks good on screen, so they add in the stunt double to make the jumping, spinning kicks and such.
Now this isn't to say that the rest of us, me in particular, don't have girth that would get in the way either.
P.S. Mark, I couldn't stop laughing when I read you comment about irony.
Luckily, I wasn't drinking anything since I'm not as skilled in the art of drink keeping like John is.