does he have an erection during REM sleep?
(e.g., in the mornings)
if so... then it is psychological
if not... then he needs to get a physical check up.

can you see a relationships therapist together or something like that? help you guys figure out what might help... can be hard to figure it out and articulate sometimes...

does he feel under pressure to perform? it might be that the initial 6 months simply is unsustainable... but things didn't fall back into a more comfortable routine... rather... he has developed some kind of performance anxiety... dunno...

how is he with cuddling and massage and physical contact that doesn't (or isn't expected) to culminate in sex? there can be other ways to feel physical closeness than sex... ways that take the pressure off...