A relationship between an openly queer person and a closeted one is more difficult than a relationship between two closeted persons. Dating a closeted person comes with many restrictions and conditions, but the love remains the same between any two people regardless of the coming out status in society. A person who hides his/her sexual orientation and plays hard not to reveal it to get rid of unfortunate circumstances is a closeted queer person. It's hard for a closeted person to deal with themselves and it is also hard to deal with their relationships too. These are the ones who can easily get targeted for their open-mindedness in the community. Yes, some hypocritical mindsets in the LGBT community point out the closeted people. Instead of understanding them and their situation, most of the people judge and criticize them. Of course, due to their hidden status, they won't get targeted by the society, but they get screwed up by people in the community who are aware of their sexual orientations. It's difficult.
Love can't be controlled. It can happen to any person and at any place. An open queer person may fall in love with a closeted one and that is inescapable, but dealing the situations between them to take it further from dating to a committed relationship, includes many things to be followed. Nothing is impossible if a person tries his level best to achieve it.
If you are an open gay/lesbian/bisexual/transgender/genderqueer, whatever, and in love with a person who is extremely closeted but deserves all your love, then you must take few ways to lead your love life in the right direction to reach a fruitful relationship.
Patience is the most important thing to deal with your closeted partner. Unlike you, he may take time to explore himself and get ready to see the things the way you see them. Do not rush and freak him out. Give him enough space to get comfortable and enough time to open up himself and yes, this happens even though you know each other very well.