maybe do the medium.. he may be in that for ages.. edens thighs change shape alot, as she moves more and stuff... i know with some of her other nappies she's been in them for 8 months and somedays they seem better fitted than others... but fitting well around the thighs is important as it keeps the toosh lined up with the stuff in the pocket the best and avoids leaks, not that the bumg does that lol