One more thing: once he has joined the ranks, if he wants to get Commissioned he has to get his chain of command to sign the forms endorsing him as suitable, not always a given. As someone who has seen several troops get Commissioned, I also think that the process is often harder than getting Commissioned straight from civvie street - OASC expects more of people who have been in the Service for a while.
The best EngOs I've worked with have been ex-rankers - but also the worst! If he's capable he'll be good, however he gets in.