Can you share a video?
Couple of things. As with horses, it could be anything, really…
The unreactive to flexions, + lame on lunge, that’s a fairly typical presentation for hind end suspensory - even in a 4 y/o. I see them often enough in young that it doesn’t surprise me anymore, especially if they are in dressage training, aimed for testing, etc.
How are his hind feet? Front feet? Good balance, is he shod, what is the angle like? I’ve seen a lot of inconsistent lamenesses caused by NPA up front and/or hind.
Stifles, typically, react to flexions - and are very easy to feel undersaddle, and tend to be the same or worse on the lunge. They can blow out through transitions and buckle, but I think if it’s serious enough to hurt the horse, that it is very obvious through feel in the tack.
I would definitely be bringing out the ultrasound. I’d also consider some things, like kissing spine and SI pain; sounds early, but both of these fit the “lamer on lunge, inconsistent hind lameness” profile.