Yeah - the surf nano stuff didn't help with speed. Made the board feel "slippery"'er rail-to-rail, but nothing gained in terms of down the line speed. I doubt there is anything that can be sprayed on that will affect down-the-line speed in any significant way on that board. Personally, I believe the real culprit is the round rails. They are more forgiving, but also add a ton of drag for bigger riders. Water will wrap around curved surfaces and round rails are used specifically to add foregiveness and control through drag. As weight increases, more of the rail will be pushed down into the wake and up towards the nose where the rail is soft/round. Creating more and more drag.
All things being equal, (wake, speed etc), you'll probably be better served with a larger board for your brother. Also, there is a phenomenon most of us have experienced, after gaining your surf legs, you are able to surf smaller and smaller baords. Probably a function of muscle memory and greater awarness how to work a board. Perhaps your brother just needs a larger board for the time being?