Aldridge is actually a good rebounder. His stats may not show it but here's a couple things to keep in mind.
1) Portland is one of the slowest pace teams in the league(fewer rebounds to go around)
2) Portland is a great rebounding team(I'm not sure if we're #1 but I know we're close)
So while Aldridge may not grab that many rebounds to boost his raw numbers, he doesn't give up many rebounds either.