If you could free work him over trotting poles- nothing high, just enough to make him notice them, that would be a good all round exercise. Very few top showjumpers school any of their horses over big jumps, they will use little jumps the horse can succeed over and concentrate on tuns and quick responses- you could do the same. If he is lunging he has no option but to change direction by turning towards you but free schooling you can turn him towards or away and practice getting him to do a turn, change, a turn and a half of the ring and change, making it a game, that way half an hour will fly by and you and the horse will stay fresh. I agree completely with Casey, tie downs can do a lot of harm- any equipment, actually, used incorrectly, can do a lot of harm. You could also start ground driving him, that is good fun too,