S.E.
Do you have a sewing machine? If you do you could custom make them with his measurements. Here is a free pattern site for trainers. Keep in mind if you use someone else's pattern, it is for your own personal use at home and you cannot sell for profit. That is stealing.
http://www.easterbunz.com/free-trainer-pattern/
There are a lot of patterns out there, there are also a lot of stay at home moms that make diapers as their business. If you don't have a machine or know where to get the proper fabrics, perhaps you could buy some already made by one of these mothers. Check out the starbunz, they look like big boy undies.
http://www.babyhopesclothdiapers.com/store/WsDefault.asp?...