Firstly, have to comment on...
Load him, drive around 30 minutes, and come home and unload him. But don't leave him in one otherwise.
Really??? I'm just confused by your suggestions & apparent ire at OP. You're going on about it being too hot for the horse to stand around in the trailer for minutes here & there, but you reckon going for a 30 minute(!) drive is OK?? Not to mention the point of the thread seems to be that the horse is not up to the stage of going *anywhere* yet.
Trailers can potentially be one of the most dangerous situations for a horse and after seeing a few serious 'accidents' one thing I will never do is tie a horse firm to something unyielding in a trailer. Instead I use a long rope and a 'Blocker Tie Ring' or such, to 'tie' the horse, but in cases of panic or accident, the rope will yield. This is also an excellent way of teaching a horse to tie safely & without them feeling trapped & panic too.
While I think negative reinforcement is a very valuable training 'tool', I personally want them to learn to be truly *comfortable & confident* in there, not just look upon it as 'the lesser evil' because you work the crap out of them outside. I also use 'lunging' type exercises as a communication tool and don't want to turn these sorts of 'games' into a punishment. Therefore I personally disagree with a couple of suggestions above.
So... I think you've got a great start, having the horse comfortably going into the trailer & standing tied with you there. He obviously trusts you a fair bit - well done! Now you've just got to teach him to trust that it'll be OK when you're not there.
I'd do this in 'baby steps'. Eg. I'd teach him to load without you in there & stand untied - drive him in rather than lead. Then go 'away' - eg out of sight - but for only a second & get back there *before* he gets worried. Lots of repetitions at this, getting gradually longer 'away' as he becomes comfortable with easier 'steps'.
If/when he gets upset/comes out without your asking, I'd allow it, give him a second to 'recoup' and then send him back in again. I wouldn't try to prevent it or punish him for it, because it is a fear reaction & punishing him for being fearful will only give him more to be nervous about. I would wait until you're sure he's confident about everything before teaching him, with negative reinforcement(putting pressure on him outside, dropping that pressure when he is in) that he needs to stay there until you say.
I agree with the people that say don't baby him(or pet or reward) when he's worried either - horses learn by association & it's pointless for your 'nice' noises or gestures to be associated with his fear. I do think *appropriate & well timed* positive reinforcement is a very effective 'tool' though and I'd personally have a pocket of diced carrot or such, to give him a bit when he's *confidently* doing the Right thing. Just be aware of what you might be reinforcing, so it's best to avoid rewarding if you're not sure he is indeed confident.
Once your horse is *confidently* loading & standing with you out of sight, then I'd start 'tying' him - again, not firm, use a long lead wrapped over the rail, a Tie Ring or such. I use a rope long enough that I can loop it through the ring & hold it while outside & put/hold some forward pressure on when necessary. One of the behavioural principles of effective teaching is that when you add a 'criteria'(such as tying the horse), it's more effective to 'relax' other criteria(such as the time you're out of sight). Therefore I'd start out getting him comfortable standing tied in the same 'baby steps' as previously. Although the more of this sort of thing you do, the quicker it becomes & I wouldn't anticipate you'll have to spend long at this until your horse is confidently standing tied just as well as he was loose. I'd do the same with closing him in.
Anyway, this is the method I've used for years with full success, be that young horses who have never been trailered or ones that have had bad experiences. It sounds long-winded & it does tend to start out that way, esp if the horse hasn't learned beforehand to trust you, but once they learn that you are going to look after them & not force them into what they perceive as dangerous situations, then training comes along in leaps & bounds!