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'm sorry, but the idea of going down on you and being near you is unpleasant. We've spoken about this a lot, and it's making me lose my attraction for you. This is a problem you can fix, and should fix for your own health. It will also result in me giving you more oral sex and initiating sex more often. If our sex life and your own health means so little with you, I don't think I can be in this relationship, I'm sorry."
And also, take showers with him. WATCH to see if he washes his dick or not. My fiance had a problem with it (much less gross though) and I just showered with him and showed how clean I needed it. And he does his best to get it that clean every time, because he loves him some head.
Deodorant is up to you. I personally love my guy's man-stink, but he doesn't sweat like crazy or smell bad outside of his pits. Maybe let him know that just because he can't smell it, doesn't mean others can't. And if he does smell it -- people ten feet away can smell it.
Beyond that, there's also the fact that I have a mental illness (PTSD).
There's a HUUUUGE difference between panic attacks and not washing your dick.
One you can change. One you have to live with.
Being in grad school isn't some excuse. Here's a fun fact -- you don't NEED to be dating anyone. If you feel like you NEED to date to be happy, you should be seeing a therapist, not the only single dude you can hunt down. I'm nos saying that to be mean -- I'm serious. You say you've been in abusive relationships -- you need someone to help you get your self worth back. You're 23. There are lots of guys out there. You're going to be out of grad school eventually. Making yourself sad just so you're not alone -- that just sin't healthy. It sounds like you need to be making friends -- not finding a boyfriend.
I know how tough grad school is. But the added stress of a not-great relationship isn't extra stress you should be adding on.
If he's amazing, supportive, and good at everything else, I take it all back. But from the sounds of it, you're with him LITERALLY because he's the only option. That is so, so messed up -- for both you and him. Please, get yourself some health. Most grad programs have free mental health care.