You didn't mention how well this horse did for your son when he was ridden. I would want to know that first before deciding if it would be my riding horse. If he was less than stellar, then my feeling is that this would not be an ideal choice for a riding horse for you. However... are you really looking for a riding horse? Could this guy just be a pasture pet? Because he will probably be difficult to sell, and rescues are typically full of horses and often only adopt horses that are in really bad situations.
As for the haltering, of course you can teach him to halter. You can teach him to come when called and pretty much halter himself. You just need lots of patience, the ability to see even the tiniest amount of "try" on his part, and an understanding of pressure / release, or reward, depending on your theory of training.
The main reason I responded to this post was to address the sunburn. My Teddy gets sunburned badly, and it breaks my heart when I see it. So I do apply sunscreen. But he always hated to have his nose and mouth touched, so it took a lot of time before he really accepted it. Even now, you can tell he doesn't like having it applied, but he lets me do it.
If you go and catch this horse, then force him to submit to a procedure that is twice as bad because it's causing him actual pain (because he's already sunburnt), then he's not going to want to be caught next time because he will associate being caught with pain. I totally understand the sick feeling you get when you see that sunburn or even blisters, but you need to pick your battles, and to me it's more important that he be catchable. So you want to reward him for being caught -- give him a treat, or a rub if he likes that, then release him. I started a story about how I did this with Teddy. This isn't how everyone does it, but it's how I did it and it worked really well: https://www.horseforum.com/horse-sto...an-mom-811183/
I was going for cutesy in the story, but read through the second post for step-by-step instructions on how I got him to be catchable. It just took time, patience, a willingness to reward that try, and a willingness to get to know him and meet him on his level.