hard to gain much and find anything that will fit your axles. if you have a manual that covers your forks or shock you can probably see where you can gain. the big worry on the front is if you get the front too low and the tire bottoms against the fender and clamps your wheel locks momentarily, but it is pretty easy to see how far you can pull those up before it hits, by taking the caps off and slowly lowering the bike on it's suspension. for the back you can back the spring off and see if it enough, but all those things make it handle pretty sloppy.
Maybe it is best to go with your cousin's fears and get him on something his size first.