I like this plan. A lot. There'll probably be a few bumps as you adjust, but in the long run I think it'll be positive. (Watch like a hawk for signs of passive aggression, though, on both sides...)

That said, I have to agree with those above who queried the mother-in-law response tactic. If it were me (and I appreciate that it isn't), I'd probably go for a combination of "I think Rob's fine as he is, actually" *bright smile* and "perhaps you should discuss that with him if you feel it's a problem". Saying "he wasn't raised to do these things" might be deliciously satisfying, but it (a) distances you from Rob and acknowledges that his mother is (essentially) in the right, and (b) leaves you open to a row, as her likely response is surely "oh yes he was!".