Gosh, he doesn't look so bad!
I'd have a go trimming him with good-quality wire cutters. Wrap him up in a big towel so he can't move, hold him on my lap and secure his head with one hand, and use the other to do two initial cuts...from the sides and towards the front well beyond the pinkish live inner tissue so you wind up with a sharply pointed and still overly long but much reduced upper mandible. Clean up the sharp point a bit and maybe the sides with fingernail clippers and that's all I'd do for the interim aside from giving him the opportunity to get out and about and peck a lot to start wearing his beak back naturally. Anytime I'd notice that the live tissue within his upper mandible had receded, I'd work on trimming his beak a little more with the fingernail clippers, then give him time again, and I bet it wouldn't really take all that long at all, maybe just a couple of months, for him to start looking (and feeling!) quite a bit better...
I sadly had a fair amount of practice trimming wonky beaks into line all this past year and a half because several of the brown egg layer chicks I bought two summers ago were intended for commercial sales and came with their beaks already cut back. Luckily, they hadn't been cut too badly and all but one grew their beaks back without too much trouble. The worst one, she still doesn't look entirely normal, though...basically she's got a slightly shortened upper mandible lying atop a slightly too long bottom mandible, a misalignment of only a few millimeters, but it does make her look a wee bit undershot. The scary part was that when her upper bill did start growing in, it started growing off to one side at first, something I was able to correct with the fingernail clippers alone. Took quite a few sessions to encourage things to go the right way, but the hen herself helped a lot because she was a very vigorous forager who would often dig her bill into the ground with a lot of force, almost as if she were deliberately trying to blunt back her overly long lower mandible herself. She also never bore me any ill will for working on her so much...in fact, even though I don't believe that chickens can think on anything but a direct associative level, I do suspect she had some dim awareness that having me mess with her bill sometimes resulted in her being able to peck and pick up things a little more easily afterwards, so she's still my buddy and likes to sit on my knee when we're outside together just because. She also likes to peck at one particular pair of shoes and pants I wear, the ones I wear when I whipper-snip the weeds and lots of tasty tidbits fly around and stick to my clothing. Peck peck peck peck peck...she's relentless! And you never know...she might get that undershot lower mandible worn back yet...