According to the morality of society these days as expressed in the law, there's nothing wrong with your relationship because you are over 16 as that is the Age of Consent in the UK. From a Christian perspective, the question is whether you and he will stay together in the long term, as whatever else the bible does say about sexual activity, it suggests that God requires it to be in the context of a permanent, till death do you part, relationship; are you his first YF or has he had a series of such relationships that have faded out?

Beyond that is the question of whether you relate together as Christians is important. We are called to make God no 1 in our lives and a marriage type relationship with a non-Christian is a barrier to that.