Yes, I would. Please take no offense but sit him down, talk to him and ask him if anyone has touched him in inappropriate ways also get him to a Dr. immediately to have him examined and be sure to bring his stained underwear with you so test can be ran of the substance in his underwear because, just my opinion, this sounds possibly like semen which could be leaking down from his anus into his underware, poolling and drying up. This concerns me tremedously as it would it imply your son may be being molested so I pray I am wrong but I seriously can not think of why else you would be seeing this from a little boy who is not witithin the age of puberty where he would be having "wet dreams" yet.