Leave it long, there are people out there who would kill for there horse to have a forelock like that. I have a dressage horse and i keep his main real short but his forelock looks like a welsh mountain pony and i love it.
It depends on what you want to do with him. If he's native & you want to do Mountain & Moorland then you need to leave his mane & forlock alone, leave his feathers alone aswell if he has them. If you don't want todo that & he has no feathers then I would probably tidy him up a bit. This doesn't mean go over the top, just tweek him a little here & there to smarten him. Best of luck with whatever decision you make