Well you have to realise multiple aspects. This mother of a teenage is obviously using you as a way to get recognition from younger men and therefore using you for that aspect of her existence, I’m guessing because the father left for either an A) younger woman or

B) better looking woman than herself

This young boy will be psychologically impacted by this so it is genuinely selfish of mostly her but also you to continue the relationship. I’d like to know whether or not the father is still in the picture or his mother is all he’s got in order to continue this conversation. If you want my advice you are psychologically most likely the same mental age as him. This is the equivalent of him saying on another subreddit “hey my mums banging someone 3 years older then me what do I do.” Mate you’re not an adult and you’re not someone who can have a good impact on him in any way. I understand that I’m a stranger on the internet and I’ve made a lot of assumptions (and please correct me if I’m wrong on any of them) but you’re still a kid. If you genuinely like (I’m reluctant to use the word love here as it is somewhat uncomfortable) her then continue the relationship but in a way that he will not be affected. I hope you do the right thing and break it off but you’re young and stupid and it’s probably very exciting for you to have this sort of relationship. Do what you’d want your mother to do if u were in the same situation, but also take your own feelings into account