Every Goodwill I've been to has at least 1 full rack about 20 feet long of nothing but suits (matched pants/jackets, sometimes vests). Should at least give it a try (but take him with to try on) since chances are good you might find something suitable (pun intended).
Maybe if he's average (i.e. an old, fat dude), but when I checked goodwill it was all giant suits that would look dated and goofy in the 90s. Any moderately fit guy that want a nice looking, stylish (slim) suit needs to buy a proper one and have it tailored. Even so-called "slim fit" at macys or Jos A bank is hilariously baggy. I've been searching for a suit for a while now and it's frustrating. Banana republic and JCew have decent modern looking ones, but they cost. I'm considering splurging on an online made-to-measure suit so I have one nice, well-fitting one. I mean it would hopefully last for a decade anyway. These cost in the $500-800 range. Some good ones I found in my research:
http://www.indochino.com/
https://blacklapel.com/
http://www.pointclicktailor.com/
Also, how can a grown man not own a suit?! Does he not meet with clients ever? Or go to weddings/funerals? Job interviews..? Even as a junior engineer I came along to client meetings and had to be able to dress properly. If I didn't own a suit that would have been a problem. Even if he's not wearing it to this party an adult male needs to own a suit.
I had the suit discussion with him last night and said precisely what you said above. He sees no reason that he needs to own a suit and absolutely refuses to buy one. He said if it's that big of a deal he would rather not go to the party. Gahhhh....he is so STUBBORN!!! I said fine, then we won't go (which was what he wanted to hear I'm sure). Later I told him I'm going to go as the date of his friend that works at the same company (he said nothing lol). Then I finally told him he can wear whatever he wants and that if he's underdressed it will be his problem. He does not meet with clients, he works for a corporation as an electrical engineer designing stuff (I don't really know what he does). Candidates do not routinely wear suits to interviews (he is on the interview panel). Most weddings we go to aren't fancy enough to require a jacket, he typically wears a button up shirt and tie though. Although a suit would not have been overkill for a lot of the weddings we've gone to in the last 5 years.
I agree that a suit can be useful and "grown up" for an adult male, even an engineer. My husband owns one suit, plus one jacket.
He wears the suit to the holiday party every other year, and to my company party also (except the last 2 years they have been on the same date).
He wears it to weddings, but the last wedding we attended was two years ago? Funerals are rare, probably the last one was 10 years ago. (My mother died more recently than that, but it was unexpected - we were already there but didn't have dress clothing with us).
He wears it to occasional business trips, but usually only the ones where he presents to government folks.
That said, honestly, dark slacks, a tie, and maybe a jacket would be fine for most of these things. I've seen engineers wearing sweaters and slacks or jeans at a holiday party. Will he feel uncomfortable or out of place? If not, then go for it. I have an acquaintance who only goes to the holiday parties that don't require "looking nice". My husband's company party is a "theme" party each year, so every other year is cocktail attire. For example, last several years have been: James Bond (cocktail/ tux), Dr. Suess (dress down OR cocktail), Ugly sweater, 40's, etc. She went to the Dr Suess and ugly sweater parties.
As a female engineer, I've done anything from a black skirt and blouse, or slacks and blouse, to my plain black dress. I get so many compliments on the black dress. It's the only dress I own.