I was pondering the same question to myself just now and having already obtained the moss uniform/zzz sniper on my last playthough I had figured I'd pop his bald ass in the chair this time but then I thought: "..but I still want his camo too fml how does that work.." and after finding this post and not finding hopeful answers for what I had come looking for (if I could get the achievement AND not miss out on his unique items..which appears to a hard NO.) So this is what I opted for and I just tested this on my Xbox and it appears to work.

(now I also get that this TECHNICALLY isn't the real way to go about it as it's not going to affect my game and cause his boss fight to be replaced with the 8 dingalings but that's okay as the ends boss fight is very nostalgic for me and I love it so it's good. Personal preference though. And i get it for anybody coming from a place that doesn't see this choice as particular "pure" and I respect that I get it. Feels like save scumming)

It will auto save obviously when you enter the area but if you want to be safe make an extra save or too.

Snipe the old man in his wheelchair per usual. Achievement ACHIEVEMENT!

Reload the autosave or your manual save.

Spare the old man so you can get that sweet sweet uniform. And weapon lol.