What's the best way to get a mystery stain out of laundry?

Everything imaginable seems to attract to my 5yr olds cloths. Just recently he came home w/what seems to be greasy spots on the of a pair of shorts he was wearing. I have laundered the shorts 4 times! The 1st 2 times I used spray n wash. The other 2 times I rubbed the stain w/hydrogen peroxide and added it to the load of laundry. My husband and I have no idea what he could have sat in. Any suggestion?