Could you maybe start at just some local "fun shows"? I know in my area, we have all sorts of shows, everything from your basic cheap back yard type fun show where most of the beginners get their start, to rated shows. If you have some nice low key local fun shows, you should start there, build up his experience under a western saddle, and your confidence in doing a new discipline.
Obviously he's not a big fancy western pleasure Quarter Horse, but I think with a little work he'd do fine in some local shows, and you can still get your showing fix. You should definitely find a saddle that fits him first, before really working him in a western saddle. If someone decides to talk about you, or laugh at you, they are just not confident in themselves, and as hard as it is, you should ignore it. You've done amazing continuing to ride, and do what you love in spite of all your medical issues, and that's something to be proud of. I used to barrel race my Thoroughbred mare in some local shows in an english saddle, as there was no way I'd find a western saddle that fit her, but I wanted to keep her from getting sour doing the same thing over and over again, as she got bored quite easily. I know that people thought I was weird, but she still kicked butt, and I had a blast, and that's what counts. He's a cute little guy, I'd seriously take him in a heart beat! I love the ponies. Good luck with him, and just cause he's so cute, of course more pictures would be awesome. Just keep working on half halting, and slowing down his trot so he starts to jog, and get him to bring his head down a little more, and I think you'll do fine. Good luck with him, and remember to just keep having fun.