Maybe he feels treating you so well, gives him the right to grope you whenever, wherever. Does he do this sort of thing when you're out in public?
I think some men never grow out of their schoolboy mentality, where they are completely fixated and in awe of lady bits. I don't know what the solution is, if you ban his behaviour, he will be like a confused mutt and not know when to grab and when not.
Maybe you'll just have to put up with it. I'd suggest grabbing his bits unexpectedly, just to see how he likes it, but I'd guess he would like it.