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.
Your reply definitely hit home for me. First off, his knuckle curve is relatively hard in comparison to his fastball. It is not 12/6, but is tight and is a little more slurvy. More like 11-5 or 10-4.
I don't have a number on it, but I'd guess it's only around 5-7 mph less than his FB. I do think it tunnels well with his FB, which is why he is so effective with it. Honestly, I think he could throw the knuckle curve 100% of the time for an inning to two and get a lot of batters out. He struck out the side of a HS JV team only throwing one FB last week. The problem is that the second he throws the FB, its like a BP fastball (which is why the catcher had him throw curveballs every pitch last week).
I also completely agree that he has trouble on the other side of the ball from the curve. He's always had trouble pronating with any pitch (or even on throws from the infield for that matter).
I feel like a changeup would really work well for him, but if it's a giant struggle to throw, that seems like a tough task. I don't think he'll ever have a dominant FB. For a kid who doesn't have a dominant fastball, but a really good curve, where do you go?
He does have really big hands. Bigger than mine already, and he's 5 inches shorter than me right now. So maybe split is the answer?
Anyway, thanks everyone for the replies. It's already been an interesting conversation.