I was in a team where this happened. Management stuck their head in the sand for the longest time, while the morale of the team started tanking, as noone wanted

I was in a team where this happened. Management stuck their head in the sand for the longest time, while the morale of the team started tanking, as noone wanted to work with this person, who needed a lot of attention and never seemed to improve, even after years of trying. My recommendation would, for his sake and yours, to let him go sooner rather than later. Working in the wrong position is also going to tank his self confidence and will do him no favours.