The real problem is that he's spent most of his life fit, being overweight doesn't really give him any insight to what it's like to be unfit and overweight for decades. He may not be able to workout as he did, but he has the base fitness that most people trying to do the same rarely have.
I admit that it's perhaps a bias I have but I'd probably be more comfortable with a trainer that had been overweight at some point. Not so much one that did it as a stunt (look at all the
PR, and their filming a documentary).