Yeah, twin skin PeterLynn ARC is the one.
Kiteboars do not need ANY float, if you plan to stand up on them. Surprisingly.
I started on surfboards and windsurfboards, and found a twin tip around 6’ long, about 15" wide is plenty for the 5-12mph winds if my kite has enough power. Thickness can easily be 3/4"…yes, less than one inch thick, and plywood works just fine.
Tip rocker, you don’t need much, as some guys ride plywood dead flat. Flat works if the board is short, so you can keep whichever nose up.
At 6’ long, you’d need tip rocker on both ends…maybe 2" max.
Insulation foam, 1" thick, skin of epoxy resin and fiberglass, maybe 3 layers of 6oz in the deck, 2 on the bottom, concave the bottom for glide and better edging.
Fins need lots for beginners, so make some of your own lexan or sheet glass, can be thin, like 3/16" or slightly less, about 3" tall and 6" base, 2 sides near the tips each end and maybe one centered on each end.
Small rounded edges…sharp edges skip out when powered, and on such a big board…6’ x 15 or 16", it’s a BIG board, small round hold better.
The board is ridden at about 35 degrees banked over.
For him to stand up in steady 10mph breezes, he needs a 18 meter kite, lots of skill to send the kite fast low, bent legs and constantly sending the kite for extra power. 10mph is NOT good, but he can stand up, ride downwind, and continually send the kite up and down for power.