How tall is he & what's his weight? 
Cooper is in a med itti, he's 82cm & 9.4kg. The small fits really well, but is rather close to his bum crack 

 med's a great fit all over in an AIO, legs can gape.
My CC is 81cm & over 13kg, med's are small on her & larges are a great fit.
Just for comparison sake 
 
Good luck in your quest, love Thirsties covers here too