I was going to suggest something similar to MummyNaomi - so you can start him out at a lower level, but as he grows you can put shelves or drawers or even a little desk under the bed, and have the bed as a bunk bed.

DS's room is pretty small, so we were looking at options like that, but ended up deciding that it's cheaper to buy a $150 bed now, and a $550 bunk or bed/desk combo later, than a convertable one now ... so that's what we're doing

But the planning is fun