Weddings are expensive. Gifts do not off set the cost unless of course it is immediate family who’d give substantial amount. Unless you were planning to give him 1k he likely will not get rich of your gift. I doubt anyone invited anyone to a wedding in hopes for a gift. Just not realistic. I don’t see how he used you by inviting you. Sure you might not be close friends but people invite all kind of guests. Not everyone is a close friend

You can’t possibly know if his marriage will work out. Age difference is their business. It’s not always an issue at all.