Wedding rings ain’t cheap, you know.
Beyond that it could be any number of things. Marriage is a pretty huge step and it’s natural that even the most confident person will feel some hesitation about doing it for any number of reasons: money worries, personal worries, and so on. For example, if his parents are divorced, that’ll make anyone gun-shy.
Talk to him about the future but most importantly, be patient. If he’s not going anywhere, there’s no need to bum rush him to an artificial goal.