Have an 18 month od filly, and I am always giving her new challenges.
Does your horse lunge really well, in all gaits, with his head lowered,
teach him how to lower his head with the halter, hold the lead rope and apply little pressure in a downward motion, if he drops his head, stop pressure and praise, if not apply more pressure until he does. continue doing this over time (may take days or weeks) until his head almost reach's the ground.
He is going to go out into this big world, so have you shown him items that may scare him,
can you groom from ontop of a fence so you are higher than him,
is he good with traffic, cars trucks etc,
plastic bags, (rub it all over him including his head)
boxes that move, (use string or rope to pull in along)
lunge outside the round yard without a problem, and with other horses working close to him,
does he follow you around a paddock, (without a lead rope attached to him)
can he back, go sideways when you ask him,
walked him over a piece of wood on the ground, like a door,
walk in between things, ( you could use two crates standing up) and tie items to the crate that may frighten him
walk through water when you ask him,
can you crack a stock whip near him without a problem.
have you used long reins on him yet. (so you are moving him forward from behind)
these are some of the thing I do with young horses, because they may have to face it when they are out riding.
good luck, if I can help you further don't hesitate to ask.