What kind of shape is his curve, big 12/6, tight 12/6, more sweeper?. I'm betting it's large shaped and that it does not tunnel well with his fastball and they look completely different to the hitter out of hand. This makes it easy for hitters to just spit on the curve and sit fastball. Some guys who have really good curves are comfortable on that side of the ball and less so pronating so they have difficulty throwing a change-up. A cutter would work well off the fastball and may be easier for him to master, sometimes you start trying the cutter and end up with a slider or vice versa, either can work. Another option would be a split (if his hands are big enough) which would also act as a psuedo change-up, don't always need to spread your fingers to insane distance to throw an effective one.
I think he just needs something to help disguise or tunnel well with the fastball. In HS where 4 inches off the plate is always a strike, something that continues to move that way works very effectively.
Pitch development requires time and lots of experimentation. It's what works for him, if you don't know where to start, take someone else explaining their pitch and change it up, experiment, change the side of the horseshoe, etc. Ie Make it his own or not.
Having a third pitch is crucial, I've seen HS guys with 3 pitches throwing low 70's and being effective and the guy throwing mid 80's with just two get his fastball hammered.