Yep, medium sounds right. L is about the same size as A, I guess. I would expect the medium to fit him for quite a while longer at this rate (am secretly hoping we'll TT before he gets to a large, but we'll see how over-optimistic that is).
I just use cheap fabric for the outer part of the nappy - any old machine will do to sew it - and the microfibre towelling for the inner/pocket insert. Actually works very well. Let me know if you want a copy of the pattern - My MIL sent it to me, apparently it has done the rounds through her family.