My 7 yr old son was in hospital with scarlet fever about 6 weeks ago. he is now complaining that his joints are sore. could this be related ?

Maybe/maybe not: Scarlet fever is just an old term for strep A infection with a red rash. If untreated with antibiotics within the first week to 10 days there is a 1/200 risk of rheumatic fever. Six weeks out would be a reasonable point for it to show up, but the joint pain is not specific to RF. An exam and testing would sort this out.

Yes: Yes. Some patients with viral or bacterial infections can get a inflammatory condition that can involve one or more joints. I would make sure your doctor is aware.