Before you drop a bunch of coin on a replacement clutch, sometimes you can modify how the clutch behaves by making minor changes to it's components.
If you're mechanically inclined, pull the clutch cover off, open it up and see how it works.
You may be able to make it engage sooner by increasing the number or mass of it's engagement system, using lighter springs or a similar method for much less than $400.
It's worth a shot.
There's really only two styles of clutches that I'm aware of with these, one is a centrifugal drum, it looks a lot like a drum brake. This type is very common in weed whackers, but you see them in motorcycles too. It uses springs to allow the partial engagement at a certain RPM and as the RPM increases, it engages more.
The other type looks a lot like your typical motorcycle wet clutch, multiple plates stacked together with ball bearings or other mass stationed in grooves in the basket to force the clutch plates together and springs to force them apart. As the RPM increases, large ball bearings are often used to force the clutch plates together. If there's open slots, you can add an even number of ball bearings to make it engage sooner or remove some to make it engage later. Be sure these stay evenly spaced, so if there's twelve slots, three filled followed by an empty space, repeated three more times for nine balls in 12 slots. If you want to remove three from this arrangement, it will now be ball/no-ball repeated five more times or six balls in 12 slots. To remove just one from the original nine, it's now going to be ball-ball-space repeated four times, for a total of eight balls. Adding one ball to the original set up, for 10 total, five balls followed by one space, repeated once.
If there's no open slots, you can't add any obviously, but you can find a denser material, such as wrought iron has a density of 7.75 grams per cubic centimeter, vs. "plain carbon steel" at 7.86 (what's likely to be in there) vs. stainless at 7.9. These seemingly minor changes would make a significant enough difference to make it worth looking into. Although SS ball bearings are going to be somewhat expensive since they're usually 1/2-3/4" diameter bearings.
Changing the springs is probably going to be more cost effective if you can't add any ball bearings, increasing the spring rate will make it engage later, decreasing it will make it engage sooner.
The third type I can envision, but have never seen, would be a dry clutch most likely, it would look a lot like your automobile standard pressure plate/single dual sided clutch disk set up. Only thing is it would have fingers that flip out to engage the clutch at a certain RPM. I can go into how I would make this behave differently, but I don't think anyone uses these, so I'm not going to. If you open the clutch cover and find one of these, just by the tomar, you'd spend a lot of time and effort changing this type of clutch's engagement RPM, it wouldn't be worth it in most cases.
You could just give him more seat time and see if he gets used to it, this is probably the route I'd take first.
My two cents.