I agree to just leave it how it is. I plan on doing dressage with my mare who also has a long, thick mane and at some point I will have to rosette it which is best done with a mane 4" or so in length but I will work around that to keep her mane long. You shouldnt have to shorten his mane length if you don't want to
There have been horses that have won World Championships in western pleasure in Quarter Horses, Paints, and Pintos (stock type) with a long mane. If you are going to leave it long to show in western pleasure you should at least band it. That will keep it looking neater. Also keep in mind that the rest of these horses' appearance was perfect and the long mane complimented their head and neck. Some horses look more polished and attractive with a short mane. If your horse has a short neck and not so perfect head shortening the mane will help make its neck look longer and compliment the head better.
If you're showing in local shows, there is no need to cut the horse's mane. The judges are looking for the proper movement, collection, smooth transitions and other criteria -- none of which include the horse's mane. I'd rather ride a solid western pleasure horse with a long mane into the ring than ride a perfectly groomed horse that is not ready to be in the class yet. Concentrate on the elements that will be judged (educate yourself as to what those are exactly for your competitions) and work on those. If you have problems placing well, try to get feedback from the judge if possible. And always consider taking or auditing the classes the judges must take, so you know exactly (from the judge's point of view) what they are looking for. Even in those classes, you'll discover that the horse's mane is not on the evaluation criteria list.
Do a solid job of prepping for your pleasure classes and leave the mane long if you wish. :) Have fun!
With local shows I wouldn't worry about it too much but you have to remember that if you nd another rider are tied for a place the judge will more than likely place the rider with the short banded clean cut mane above you. My mom was a judge and making sure your mane is cut and banded is a BIG thing for her. I show WP and in my opinion a long mane looks sloppy and unprofessional for a show pen unless your a reiner. Don't just leave your mane long try to braid it somehow.
Leave it long if that's what you want a friend of mine cut her mares mane for western pleasure abut 5 years ago (it was past her neck) and now she wants to kick herself in the butt cause it doesnt grow more then 6/7 inches long regardless now.. if your only doing local shows it shouldnt really matter, major showing would be diffrent but I recommend going all natral .
For the local circuit? You can keep it long.
Its going to be more work. Make sure the rest of the horse is scary perfect. Get him clipped. Ears, bridle path, facial hair...just everything. The mane will need to be silky soft and lay flat. I would put a mane tamer or something to flatten it. Band it.
There is nothing wrong with a long mane...but the horse sure heck better have the chops to back it up.
If you like it leave it. There are some people at the APHA level that show western pleasure with a long mane. At the APHA world show one of the top ten two year old western pleasure horses had a long mane.