I reallly rather like him for dressage and some jumping. I think I see a gag bit in his mouth? That would explain the "set" of his neck--not liking that for dressage or any flatwork; however he has 3 nice gaits, though he's not being asked to show much. The canter is quite nice with 3 clear beats and is his most uphill gait, the trot as with the canter needs impulsion. He has a nice overreach in the walk and a soft swinging tail at trot. The tactful rider sits very lightly - almost half seat at canter, and seems to me to be working to present him as quietly as she can, hence the quiet rhythmic ride at the cost of impulsion. Considering how OTTBs look when coming off the track, I think his basics are well started for the most part.
GP material? Probably not - though you can say that about 95% of purpose-bred warmbloods out there. Flashy, sound, well-started with 3 decent gaits? IMO, yes. And remember, you can train a trot, but you have to buy a walk and a canter
The rider looks well on him-how tall is she?
A 4 y o who races in between dressage training? That's kinda cool, and he looks sound in the video. IMO not 1st L-ready, and not any level-ready in a gag. But I like the potential I see here, and quite like what the rider has done with him so far, including the rudimentary turn on the haunches. I feel like the rider/trainer has a reason for working him this way, and I'd love to know why he's in a gag versus a simple snaffle of some sort.
And by all means, do consider eventing!