Boat for father and post-Topper son?

Flying 15 is a different kind of sailing. Being a keelboat changes a few things, like it's much harder to get out of the  water, less trivial to tow to another venue. The racing is different, often closer, but not the thrills of getting the kite up on a 400. Annual costs likely higher? In keelboats, I think small differences in boat speed can be magnified, so you may want your boat to be as good as the others, while a 400 can get away with not having new sails if you sail it well? Well worth asking for a ride or two. Just be warned it's the first step down the slippery slope of 'leadmines' which can get expensive,,,,  :-)