Eye contact is politeness, it’s just showing that you’re engaged in a conversation or aware of someone’s presence. There’s nothing meant in it, it’s just polite.
Eye fucking is when you both know this could go somewhere. I often say it feels “nuclear-tinged”; it could go to “DEFCON1” if you know what I mean — eye fucking could lead to actual fucking.
Eye fucking is also sometimes used aggressively. Case in point, I have a neighbour who is quite clearly hot for me, but he’s also manipulative and likes to play mind games, including jealousy games with a fake throuple. He likes to maintain eye contact, kind of as a dominance move, so I eye fuck him back — no, I’m not going to actually sleep with him (I don’t want to), but I’m putting the thought of it very firmly into his mind. I know he imagines it, and I’m making him imagine it more. Why? Because imagining it is as good as he’s ever going to get, but also because by making him imagine it (instead of getting jealous and begging him to date me), I hold the upper hand. Maybe some day, he’ll actually get the hint.
I should say here that I enjoy mind games, but the right mind games — I enjoy strong eye contact (eye fucking) and word play. It’s a kind of foreplay for me, long before you even get to the bedroom. I’m quite an intense character, in a way; I intimidate guys who aren’t so sure of themselves - I dress well, I’m intelligent, confident and articulate. Hence, they try and “bring me down a peg” by playing mind games. It never works out for them.
Where it can become creepy is when it’s not wanted; when you’re eye fucking someone who, essentially, merely acknowledged that you exist. In that situation, you’re now staring at them and creeping them out. Don’t try and eye fuck someone who isn’t trying to stare into your eyes, you’ll come across aggressive, creepy and rude.
Now, let’s talk about your husband.
First, I’m sorry to hear that he’s going through health issues. Health issues can affect confidence and performance, including - and especially - in men.
As @JGood said, writing a letter can be a huge help. It does sound to me like his confidence has been wracked and he feels like he’s not goed enough for you, and any time you bring it up, he sort of sees it as though you are “pestering” him.
You say there’s no intimacy, forgive me, I don’t think we’ve spoken before (I don’t think I’ve been here a year yet) so I’m not familiar. How about things like hugs? Kisses? Also you said he intiated sex, so he clearly has some desire. It sounds like it’s about making him feel desirable.
Good luck, and do let us know if we can be of more help 
For brevity’s sake, and using @JGood 's example, eye contact ends at two Mississippis. Eye fucking is when you say “fuck the Mississippis” 
