I think you need to separate things out here, because you're covering a lot of ground -
1 - you are grieving because of the loss of your fertility, and the loss of the children you are now not going to have, and that's bringing up all sorts of regrets and "what if's". That's a legitimate feeling, and something you will need to work through, maybe talking it through with other people who've gone through an early menopause. It's going to be a difficult time at any age, but it must be specially hurtful when you were trying for another baby, as most women have had a chance to get used to the idea of no more children before their body makes it definite.
2 - you are frustrated that you are making most of the running for having other children over - it's not just mums of onlies who experience that, but I think we're often the ones with a bigger "investment" in the idea of our child needing to have playmates/buddies and feel under pressure to provide it. If the kids your DS likes are happy to come over, then does it really mater that they come to you all the time and don't invite back much (or at all)? They may have parents out at work, or siblings cluttering up their house so they can't focus on playing with their freinds when they have them over. Or you may just be good at being welcoming! As he gets older, it'll be more up to him to invite them, or make arrangements to go out together, so it will be less on your shoulders.
3 - I thnk you are massivle romanticising what its like to have a sibling - many cildren don't have much in common with their siblings and don't enjoy their company that much. Some really hate each other and send most of their time arguing. Even if you had another boy of similar age, there's no reason to think they'd be off on bike rides together.
4 - Which of you is unhappy about the situation? You or him? You say you don't even know if he's happy - I think you have to watch out that you're projecting your own unhappiness onto him. He may be fine with current friendships - why not ask him? If he wants to spend more time with people his own age, then there are loads of clubs - Scouts, football, music, drama, all kinds of things where he can spend tim with others. He may or may not make friends and meet up with them out of the club too, but that doesn't have to happen for him to have fun.
5 - finally, please stop thinking of this as being a terrible fate, or you being a burden on him in later life - that sounds like depressive thinking, not a proper assessment of the situation. Onlies can be just as happy as anyone else.