If he really is suffering from depression, his behavior is typical. If he is struggling with his sexuality, that would just make the situation worse. I know it is hard work being the only one in the relationship who puts forth the effort, and it gets old really quick. But if you really care about him, I wouldn't write him off.
Depression is a serious thing, and for the person suffering from it, it can be overwhelming to try to maintain friendships. How can you maintain a friendship, when you can barely get out of bed and function? That doesn't mean you don't care. It means you can't figure out what you have to offer to anyone else.
If you care about keeping him in your life, you need to sit down and talk to him, really really talk to him. Tell him how you are concerned about him. Tell him how much he means to your family, and how they are all concerned for him. Tell him you want to help him get better. Don't tell him that he lets you down. Ask him what he needs in the way of support from you. Maybe get some literature on depression and bring it with you. There is still a huge stigma attached to depression, especially for men. Women are more comfortable talking about their feelings and it is hard enough for us. Get some info or some personal stories to share with him about how it is NOT HIS FAULT. It is a chemical imbalance. If he was diagnosed with diabetes or something, he wouldn't refuse treatment right? It is the same thing. Encourage him to see a doctor. If he really is suffering from depression, he needs all the friends he has. I wouldn't even mention the possible boyfriend. If he gets better and gets the depression under control, he will probably be able to come to terms with that and with telling you a lot easier.
Depression hurts more than just the person suffering from it, it hurts everyone around them too. But if he depressed, he is not intentionally trying to hurt you. He probably doesn't even realize what he is doing, or if he does, it only feeds into the depression to make it worse. Be his friend and encourage him to get treatment. Good luck!
*Added*
As far as his contact with your family for the foreseeable future, I agree that not allowing him to make promises he might not keep would be best. Young children don't understand that and it isn't fair to them. If they are old enough, you can certainly try to explain things to them. But I like the idea of letting him know about your family's plans without announcing he will be coming along. Then if he makes it, great, if not, the children aren't disappointed. I have friends that I have done that with in the past- just let them know and don't count on them, and no one is disappointed.