RE: I sonic 145 VS F-Type 138
Notwithstanding my earlier comments, I have ultimately found that some folks aren't well suited to be windsurfiers, and it could be that your brother might be one of those folks. My brother didn't find windsurfing viable or interesting overall, so I had to be more understanding of others and their wishes. While I'm not saying directly that your situation is the same, it's something to consider thoughtfully.
I can easily say that most of the boards that I have bought over the years were immediately awesome, and surprisingly easy and rewarding in nature. Despite my positive experiences, there's that chance that a targeted design isn't well suited to someone's desired needs. That might require seriously considering a different design/product focus.