there is no option of an FSIQ.
Same here... my son's ADHD made him too difficult to test (age 7) and the psychologist couldn't calculate his FSIQ.
He was potty trained for poop entirely by 2.5, but pee has been an ongoing problem.
This is like my son, only the opposite: he was done with pee around 2.5 ish but poop remained a problem for years (we're passed it now - he's 11). For him it was a combination of sensory issues with hyperfocus - he'd ignore the signals from his body until it was too late and then he'd be rushing to the bathroom... and changing his pants.
Anyway, he has outgrown it as your son likely will too. It's rough while you're going through it though. If it's not anatomical but behavioral instead they reach a point where they simply want it to stop enough to pay attention and deal with it. We tried everything... and nothing we did mattered... it was all him, when he decided he'd had enough (ie embarrassment - it bugged him enough to make it a priority and pay attention).
