Welcome, Zepol.
I'm not sure the memory part has anything to do with personality type. It really hasn't come up as a topic that I can remember.
Oh, I see what I did there, but I meant it straight, not as a joke. My memory has always been excellent and it's my impression most INTPs are normal or above average. If your son is having issues, there may be some other cause - it is a new thing?
As far as completing, well, it's more or less a problem for INTPs, depending on how well they are aware of their tendency to dawdle, explore and analyze. If they're aware, and care, things get done.
I hope you find what you need here. And your son would be quite welcome, you know?