I feel very scared & guilty my son had an untreated strep throat that went away 6 weeks ago. How long until it is completely behind us I worry everyday.?

Hard to say. The earliest problems with untreated strep often come up within the first 2 wks & effect the kidneys. Rheumatic fever/heart & brain effects are more likely to be 4-6 weeks out from the first fever. Either way, as you pass the 6 wk marker without known issues you may have been lucky. The estimated attack rate for major complications of untreated disease is 1-2%. Subtle effects may appear later.
Ask your doctor . There is no way to know without doing appropriate tests. I would highly recommend you get your child attention immediately!