Initial reaction is to say "Make sure his life insurance is all paid up."
Even nine HP w/prop is enough to kill you if it hits you in the back of the head.
I know that is not helpful, but in all fairness to anybody that might respond, you need to supply more information and/or pictures.
Engine mounting is among the most important parameters in the question you ask, and there is NO info pertaining to that given.
All I can say is that if that engine/prop is only bolted at the bottom to a pvc framework, you couldn't pay me enough to sit in front of it.
Just a quick short list:
How long, wide and deep aft are the pontoons?
How far apart are they intended to be?
How much does the engine, prop and guard weigh?
What is the calculated freeboard with Dad included?
Are the pontoons going to mummified with 'glass?
Are the 2x4s unfinished outriggers to keep it upright?
If so, what will they look like? If not, I believe it to narrow to turn without issue.
With no more information than supplied here, this looks like the beginning of a
game of a thousand questions that many people will not take the time to play.
It is not my intent to belittle the effort, far from it; it looks like cool little gig to me.
There is simply not enough info supplied to confidently provide good & usable constructive feedback.