I have built a few boards in the 7 - 9 foot range, 3 1/4" plus or minus, which is very thick by general standards. I’m guessing by your response to my previous post that you don’t feel they would work for your brother, but I gained a few insights I can share on the thread topic, take them for what they’re worth, or maybe they will just aid to keep the conversation going, at any rate.
First off, the obvious fact is that a more buoyant board is easier to paddle, in flat water. However, I have noticed that in catching a wave, as the water rushes up the face, you are fighting the board’s buoyancy to get down the face for the initial takeoff. At that critical point, I have found the increased buoyancy makes catching a wave more difficult. You can see this with SUPs situated in what appears to be the ideal spot for catching the wave, paddling hard, and getting passed up by the wave. Some people have found they couldn’t catch waves with my thicker boards.
What I do is use the flatwater paddling advantage to build speed before the oncoming wave gets to me, so I don’t just flip around and go like a shortboarder. When catching a wave, I actually feel like I have to paddle a little harder and faster than I would on a less buoyant board. Once I have the wave, I try to take one or two more strokes before popping up.
The rest of the time, like paddling back out, the volume makes it easier. I don’t attempt to paddle through breaking waves. I surf point breaks, and always paddle around and over to the lineup. If you have to paddle through breaking waves, once again you will find yourself fighting the volume.
I have also found that thicker boards are less sensitive to weight shifts, so minor adjustments on critical takeoffs are less effective. You kinda have to ride them a bit like a longboard, even if they are not a longboard, because they have the foam of a longboard. As a general rule, you can’t surf the rail like a thinner board.
I am 5’7" about 185 lbs., so if your brother is closer to 300 lbs., like big Jimbo Pellegrine, (2nd pic below) then you have to adjust up. Big Jimbo rode a “shortboard” (shown in the 1st pic below) that was about 7’ long, 24" wide, and 4" thick, IIRC (before the accident, don’t know if he is still surfing since losing his arm).