As for love at first sight... yes its possible. It happened to me with my first lover. He smiled at me and my heart completely melted... but I was 'confused' back then so it took six months for us to take it to the real relationship mode with sex and my complete acceptance that yeah I'm queer.
He was patient yet persistent, telling me almost right from the start that he too fell immediately in love with me the moment he saw me.
Long story short, he ended up going to prison for a very long time and let me go free - a 25+ sentence was too long in his mind for me to wait for him.
I think that what he and I had was a soul-mate connection. Thus that immediate love and the persistence of that love I still carry a torch of sorts and miss him dearly at times. And that is 24 years after first meeting. I learned he died recently, that seriously did a number on me.... So after all those years, after 5 other men there was still 'something' that connected me with him.
I don't think love at first sight is all that rare... It think that biology makes or breaks a relationship. Real chemistry, real biological attraction can and does exist between people. Things like the scent of a man will make or break your desire for him.
Compatibility can be anything... If you are not prone to throwing yourself at any man that walks into a room, then its a good chance that there is this biology and compatibility between you two.
Actually the HIV is a big deal.
He has already said he doesn't want to pass it on to you. That means sex is going to be at best 'strained'... Most likely it will peter out far more rapidly in a regular relationship.
My ex of 14 years came home with a positive test result in the second, near third year of our relationship - while we used condoms and I was the top, he didn't get into it, rarely got off and steadily grew distant from me, complaining of 'headaches' and being 'tired' when I wanted to... you know.
He professed he didn't want to infect me... And other things, like minor cuts with a bit of blood I would pull out the first aid kit to do the nurse thing and he would freak out... This is more of the 'I don't want to infect you....' thing going on.
So it is a serious thing for some HIV pos people.
There is couple's counseling... There are mixed status couples who make it work. They work it, and they have a good understand what the risks are. Most I fear allow the third partner in their relationship (HIV) drive them apart.
You told us how he feels due to the HIV... ask him what he feels if you were positive, or he was negative... my gut tells me he already cares for you a bit more than he may be willing to admit. That whole 'I don't want to take a risk and infect you' thing - does he apply that to any hook-ups? Or will wearing of the condom suffice with other guys he meets?
Back to the HIV... I strongly suggest you both sit down together with Google and look for forums and sites that deal with mixed status couples:
Here is a kick to the head... or a shove in that direction for ya:
https://www.google.com/#q=mixed+status+couples+hiv
Since it is a thing, you two need to be willing to at least study up on the risks, consider how the dynamics of your three partner relationship will work. I cannot stress this enough, HIV is a third partner - a silent partner that has its own wants/needs and makes demands on the couple.
Ignoring it does not work. Silence does not work. Facing it, and meeting the challenges and a healthy acceptance for the potential that you will be both positive - accidents happen must exist for that relationship to survive.