Consistency is the key here, your lo needs to get used to the fact he can move & it should get better once it is not such a novelty, if you get too involved, you may make your lo forget the skills he already has.
there is something you can buy called a "safety sleep" or something like that, which I heard someone describe as a sleeping straight-jacket, but I think the best thing is to help him through this develpment by helping him get back to sleep in other positions... they still move around the bed at aged 2!!