Cats are different. Some breeds mature sooner than others.
I'd expect him to start doing it later, since at 7 months of age, he seems to think he is still the little kitten who gets taken care of by others. Our 7-month-old ragdoll is the same kind of messy kitten; living the careless life of a young single male. We are just simply waiting for him to wise up. Though one of our neutered adult males grooms the lazy youngster regularly, so he doesn't get too smudgy.
When you are brushing him, with cleaning him in mind, brush him like cats do, with rather short, "decisive" strokes. It should feel different than your normal petting. He may try to wriggle his way out from your hold, but that is normal for kittens, and an adult cat won't let a dirty kitten leave so easily. Of course, you will have to let him go if he is really going, but not at his first wriggling.