Hi, my 10 year old son has developed a painful rash with what appears to be small blisters and red spots on the back of his mouth, no other symptoms?

Painful rash. He may have the start of hand, foot and mouth disease which is due to a viral illness. Check his hands and feet over the next few days. To sooth the pain in his mouth you can give him lots of cold things like popsicles or ice cream. A combination of benadryl (diphenhydramine) and peptobismol may also help in psin control.
Probably viral. Some viruses can cause blisters in the mouth especially in kids. Because of the location of the lesions and the fact that strep throat is a common ailment it would be a good idea to schedule an appointment with the pediatrician especially if he has a sore throat and fever develops.