Actually, by age 11, some kids are on their second round of braces as part of a multi-stage treatment. My girl had her first set (upper front teeth only) at about 9 for around a year or less, and now, at 11, just went into her full-on, all-teeth set and is about to get a new appliance inside her mouth (to correct specific teeth now in the wrong locations).
If you wait longer to even start your son on treatment, it will only become more difficult for him.
Be aware, though, that estimates like "It will only be two years" are just that -- estimates. Things change and as the teeth shift, the orthodontist may find that things just need a little more time, or your son's growth spurt may come sooner or later and affect his mouth's growth too. But it's MUCH better to do treatment in the next few years than as an older teen or even an adult.
As for pain -- I don't know if you had braces as a kid. I did, and it was a very, very different process from now, with very different types of braces than are used now. Yes, there will be some level of pain, mouth soreness or a headache after an adjustment, etc., but the process and appliances are nowhere near as painful or invasive as they were years ago. It's still a mouthful of metal, of course, but overall it's not what it was when I was a kid. I hate that my lovely girl has to go through it but it is for her oral health. We're going to have an extra year of treatment because of her two teeth that are in the wrong locations, but in the end, it may well prevent her from having serious bite and jaw issues as an adult-- and she gets that.