I am currently shooting around 330 fps from one of them thar dang fangled wheelie bows. I've been shooting them for 25 years and tried a lot of different bows and heads. Todays bows are more finicky with broadheads flying at the higher speeds than they used to be. This is a big reason why mechanicals have become so popular. For fixed blade, I find the shorter broadheads more accurate.
Mechanicals do seem to find an edge for accuracy, but it's an inch or two in a group at 30 yards for me. Not enough to worry about for most hunting purposes with the exception of turkeys. But I do find them to be on the weak side. I can't bring myself to take them for moose or bear and only take a fixed blade for them. Simply put, nothing can go wrong with a quality fixed blade in itself. Not to say a mechanical wouldn't work, just my own lack of confidence in them for large game.
To your son, first things first. Tuning goes a long way to get any broadhead to fly well. I spend a lot of time getting my arrows to fly true. But it's this technical part why I like wheelie bows over a bent stick
Teach your son the basics of paper tuning and get it shooting right before trying any broadhead. (There's a lot to be said in that.) From there it's a matter of personal choice and gaining experience.
For me, the best broadheads out of my last two bows were narrowed down to 125 gr. G5 Strikers for a fixed blade (moose, bear, deer). I shot 100 gr. Wasp Hi-tech hammers for years and never had an issue. I use 100 gr. Rage 3 blade for a mechanical (deer, turkeys). I have yet to try Grim Reapers and they will be next on my list to test out.
I'm sure you already know this, but the best broadhead for your son is the one that is razor sharp and accurate from his bow that he is confident in.