How would you phrase this exactly? I'm having a hard time with it in my head...like, if he were a true slob, I wouldn't have a problem. But it's not like his hygiene is getting in the way of the major parts of his life (school, work, etc), so I can't fall back on the "you need to learn to take care of yourself" explanation.

I agree with /u/hyperbolic_pancakes when s/he says that sex and romantic relationships are a major part of life. I'd also suggest that if his odor is bad enough that you can smell it while you're sitting beside each other, fully clothed, his classmates and colleagues can also smell it, and he's going to be (or already is) That Smelly Coworker that nobody wants to be around and HR has to step in and speak to. That's not insignificant.

How I would phrase it would be something along these lines:

"Baby, you know I really like you [or love you]. You have a lot of rare and wonderful attributes that I really value in a partner, like X and Y and Z. That's part of why I want to talk to you about the hygiene matter. I've tried to tell you that your smell and laxness about showers is problematic for me, but you don't seem really to be responding in a way that resolves it for me. [Then talk about how the smell lingers, and that it smells really foul, that you initially thought he had an STD, that the smell stays on YOUR stuff so you can smell him even when he's not around, tell him some of the stuff you've described about his smell here.] Babe, I really don't want a problem so easily resolved to come between us, because I really like our relationship, but the hygiene problem is making it difficult for me to want to be close to you, and that isn't good for our relationship. Do you think you could be more attentive to showering daily, even multiple times a day, and wash your clothes more regularly (or let me wash them for you), and wear deodorant? This is a big problem for me, and I would really like it if we could fix it together."