Ok, first - welcome to the real world (don't interpret that as snarky)! Of course your relationship has been stress-free up to this point because all has been well with the world. I'm not discrediting your relationship or suggesting you haven't seen your fair share a difficulties over the years. But now it's real. A real marriage is imminent. And now the problems you face have real consequences that impact BOTH of you, colletively.
Regarding his education - I graduated college 4 years ago and still have NO CLUE what I want to do career-wise. I have a crap job, with a crap boss, with mediocre (at best) pay. I still go to work every day, bring home a paycheck, and contribute to our household. I've spoken to my husband more times than I can count about my situation. Should I go back to school? Should I try to find something else? He supports me, his trusts I will make a good decision that will be mututally beneficial. It might take time for your FI to find something. He may hever find something. Or he may have to return to school, get his doctorates and become an English professor. Are you willing to stand by him while he tries to sort this out? (There is no right or wrong answer by the way).
His job - give the guy some credit! Manual labor is HARD WORK. And he's working, period. Similar to me, my husband was working a crap job for a while. He hated it, he pissed and moaned about it daily. And I thanked him, at least once a week. "Thank you for what you're doing." Is it something he should have been doing anyway? Absolutely. But I feel it's my responsibility as his partner to encourage him and support him and express gratitude.
Potential employment - Assuming he's been actively pursuing employment, help him out. Look up postings in related fields. Or find anything you think he might enjoy.
The sex -sure, lack of sex sucks. But it's not going to be the last time you hit a low. Your sexual relationship will have peaks and valleys. Between stress and physical pain, I don't blame the guy for his disinterest. You have to learn to not take this personal.
I get it, it's hard. I've been through/am currently going through it (at least career-wise). It's tough. But life will not stop throwing curveballs your way. Outside factors will impact your relationship for its entirety. It's those challenges that prove a person's true charater, it's those challenges that test the strength of your partnership. Look at this as an opportunity to really grow with each other.
(Most of my advice is based on giving your FI the benefit of the doubt, assuming he's an otherwise good guy who has fallen on hard times.)
I would at least postpone the wedding for two reasons - 1. You want to make sure your financial house is in order before marrying and 2. You want to make sure he's the guy for you - now is the time you will learn more about who he really is, how he handles life, and whether or not you see him as a lifelong partner.
Wishing you the best of luck!