The iSonic has the same delicate construction as a modern formula board but is smaller and won't plane as early or get as good VMG in light 6-12 kt winds. So I think Jack would be better off with a Formula Experience 160; starboard's F160 formula board in tufskin construction. Either that or a Futura 155 or GO 155 / 166.
Another route would be to get a wide, early-planing board that will handle a 10.0, but that the family can use, too. Like a pre-2008 START or RIO.