don't really know the answer but just wanted to ask if he has had any ear infections/problems? I know my sisters oldest had numerous ear infections and when he finally got gromits he improved greatly (he was 7 so it was his schoolwork that improved), another friend's little boy had grommets at 2 and 1/2 and he has finally started to talk - like you said his comprehension is fine he just wasn't verbalising.

Goodluck with your appointment.