I think Size L is ok. My daughter uses Superdry and sometimes uses Premium. Premium is larger than Superdry, but the difference is not too much. So, I think it is ok for your baby to use L (Superdry) when he is using M (Premium) now. For XL, it will be too large kar! :lol:
My daughter changed to use size S 4 weeks after she was born. Little baby grows extremely fast kar! Don't worry..... and don't blame your husband ar. :-P