Give him a broom handle and invest in some golf-ball sized wiffle balls and practice as often as he wants. If he can hit those squarely (still teaching proper mechanics), he'll have no problem hitting a baseball with ANY sized barrel of bat that he feels comfortable with.
As for wood, the younger they are when they start, the less worry about transition. My 12 yr. old son and 9 yr. old daughter both use wood, they actually prefer it, though it did take a while for my son to get used to hitting with the wood (he'd been using aluminum/alloys since he was 5 and playing t-ball)...my daughter's only used wood and the aluminum/alloys "feel weird" (her words) to her now that she's tried using them.
Length/weight (wood, aluminum or alloy) doesn't matter as much. Have him extend his arm out to the side, horizontal to the ground, put the bat in his hand so he grips just the knob, holding the bat horizontal from the ground - an "extension" of his arm. His body and arm should form a 90 degree angle. If he can hold it like that for at least 30 seconds, it's fine. If you want, you can keep going up in length and/or weight and once he can't hold it for 30 seconds, that's the length and/or weight that's too much for him right now, pick one of the others. After the correct length/weight is chosen, keep working on his mechanics.
"There is no logical reason why girls shouldn't play baseball. It's not that tough. Not as tough as radio and TV announcers make it out to be ... Some can play better than a lot of guys who've been on that field." ~ Hank Aaron
"Hitting is timing. Pitching is upsetting timing." ~ Warren Spahn