I don't think anyone forgot to teach him, he's just brutally bad at the free throw line - seems to affect some big guys.
I think he's fixable, though. His misses are not in the wrong zip code like some of the lost causes are. (I think Arinze Onuaku hit less than 30% one year.)
Worst free throw shooter I've ever seen who doesn't have the excuse of having to collect 7' of assorted arms and legs in his motion was Doug Gottlieb.
A guard who can't hit 45% of his free throws isn't a liability, he's a plane crash.