They're very slow growing but will reach 12-16" eventually. You're looking at a very long time before that happens though as I was advised by some owners that they grow on average 1"-3" per year.
They eat bogwood, so they need plenty. They also make a hell of a lot of mess compared to other ples species, so be prepared for at least twice weekly gravel vacs. Because of the waste issue, they need good filtration too.
Panaques are strictly vegetarian, there is evidence to suggest that protein in their diet shortens their lives, so that can be an issue when housing with other species.
What size tank is he in currently (sorry, don't recognise the make)? Because of the water uality side of things, they need bigger tanks than you think for their size.
Hope that helps - he's lovely!