It sounds like he bought the ring for when he proposes because he wouldn’t have remembered which one it was.
But if you think you should be talking marriage by now then have the conversation.
As there aren’t going to be any children then it’s possible that he just hasn’t thought of it beyond he wants to spend the rest of his life with you, which he currently is doing.
FWIW me and DP talked marriage around two years into our relationship, went out and bought a ring and got engaged.
And then life sort of took over. Serious illness and logistical reasons why marriage wasn’t possible, and 5.5 years ago we’re still together but marriage has never happened, and yesterday he told me that he’d told a friend of his when he asked when we were getting married that the marriage bit isn’t that important, that obviously we’d like to be one day but that in the scheme of things it’s not the be all and end all.
There aren’t going to be any children, we’ve both been married before, at the moment we can’t even live together for logistical reasons, and we’ve been together so long now that marriage would just seem like something to do rather than a big statement, iyswim, so it doesn’t bother me that he now feels like that. In fact he was the one more thinking about marriage and now he’s the one who has backed down.
Is it possible that now you’re living together, there aren’t going to be any children, he doesn’t really think of it as necessary?
If it’s something you really want then I would talk to him about it.