Question: I can understand if he has trouble writing competently and that is why he has never been given his own series(although if it turns out he is also a gifted writer, that'll just make it worse), that could be a block on him, but how come he hasn't signed up to be in a writer/illustrator combo?
Like, even if you say "he has no style of his own" or whatever, he can replicate quickly and effectively the styles of every author he has assisted before. Sign him up to draw for someone who can already draw but has a hard time fitting a weekly schedule for example(not something uncommon at all) and have him on as the main illustrator. Treat the author the same way you would treat him if he was only the writer and boom, problem solved. If he is that insanely competent, he'd even decrease the workload for the editors