I received this sewing desk from a craigslist ad in the freebie section. If you have never checked out the free section, I recommend it.
I primed it then painted the body black to match his room. I took out the top of the sewing machine section. It took a bit of searching, but I found a box that fits into that space to hold the legos. If need be, you can build a lower lip from underneath the cabinet to make the hole smaller.
Now all the legos can be in the box and in the drawers and lot on the floor. He has space to the left to build and can easily dig through to find what he needs.