A flywheel wt will not reduce the power, but it will spread and smooth it out. All the power will still be there, just not all at once.
I use a 13 oz on my yz. The easiest way to reduce the power is to lower the compression and retard the timing. Some bikes you can stack 2 base gaskets between the jug and the cases to reduce the compression. retarding the timing should not be that tough do a google search for how to.
I would just put the flywheel wt on a ride it much more. Your dad will get more comfortable on it in time.