My oldest started wearing glasses when she was three. Her first two pairs were metal, and she didn't break them (she's very gentle), but they were both uncomfortable, both in sizing and in the little plastic/rubber (whatever) nose pads. I had them readjusted more times than I can remember, and they just never worked. The only reason I got her metal the second time was because the lady INSISTED that kids break plastic glasses even if they are gentle and they don't fit well, blah blah blah.
Fast forward to age 5 and our first pair of plastic glasses. Finally, glasses that fit well and don't pinch behind the ears, and don't leave marks on the bridge of her nose. I've had to get each plastic pair adjusted at about the six month mark because they do loosen with time, but that was a quick two minute (and free) stop.
My office gives free repairs or replacements (of same pair) for a year after purchase, so you may want to shop around a bit.
I have never had anti-scratch or anti-glare for her, and I haven't seen a situation where I would want either.
For sunglasses, my husband bought her a pair that is made to fit over glasses (not the attachable kind, because you can only use those with metal frames typically, but they literally just go on over your glasses.) They work very well, and you can get them for around $10 at Wal-Mart.
You'll have to use your best judgment on some...only you know how active your son is, etc. :) Good luck!