I would be surprised if we sold him, he's rated very highly by the club.
There was a story going around that Pochettino wasn't impressed with his attitude upon returning from the U20 World Cup(where he was one of the standout performers), apparently Onomah came back a little too confident and that was one of the deciding factors in allowing him to go on loan. Pochettino also seemed narked at constantly hearing people suggest he hadn't been playing him in his best position which all throughout his youth career is central midfield, I think there was a case of him saying to Onomah to go and prove him wrong but as we know, so far Bruce has pretty much used him the same way - in fact he's playing even further forward with you.
I do still think that you're not even seeing half the player when he's asked to play in the final third, he would show a far higher and more consistent level if he was played deeper but as long as he's playing and contributing positively then that's the most important thing.