HELP SINGLE MOM Testicle Size Question?

My son is a high functioning autistic. As I was getting his bath ready tonight he came in a few seconds early undressed. Usually he doesn't want me to see him. I noticed that one of his testicles is nearly double the size of the other one. He has not had any injuries that I am aware of. It looks like it could be swollen. I hate to be one of those moms who runs to the dr's office for every little thing going crazy to be told that it is nothing. I am wondering if this is something to be worried about or not. Thanks to all who answer.