Thanks for all your replies ladies
Think I'll keep an eye on it and get him out in the sun a bit more (when there is some!)
In the past few weeks he has taken to shuffling on his knees to get around (today it was with a flower garland around his neck, a hacky sack in one hand, piece of toast in the other) so at least I know his muscles are pretty strong. Might look at getting a few slimmer fitting nappies as well and see if that makes a difference at all.