Hello, Sy, and
to GaySpeak.
Wow! That's a story that can't be comfortable living.
It seems to me that you need to talk. You know it's got to happen. You need to talk to him because he's going to be confronted with choices as you are facing them yourself now. Is the boy so unaware that there is a problem?
So, it may not be illegal to be in a relationship with a 17 year-old in the UK, but I feel that you are seen as a father figure to this young man.
For some reason, I get the impression that there is no father around. Am I right? You are the older male here, and therefore the trouble you need to worry most about is the boy's mother. Unless she knows you are gay and trusts you, because maybe she knows that her son is not into girls after all, and she'd rather it were you? But I sense that you know you're not being honest with her, he's not being honest with her and he's not being honest with his girlfriend either.
There's a lot of fooling other people going on here, and you'll only be happy when things settle into a manageable pattern, but you've got to prepare yourself for denial and rejection, as the young man finally makes choices. If he chooses you, you ought to be happy, if his mother doesn't start blaming you for turning her son gay (of course, you didn't), which is how she may feel.
Therefore, you've got to have the talk with the young man. Tell him, as honestly as you can, what the options are. He may not realise quite the tangle into which he's got himself now. His girlfriend may seem like a safety jacket for the moment, but it's all going to explode one day if you're not careful.
Have you thought of a way out of the relationship, one in which you have some type of control? How soon till this young man becomes eighteen? It won't settle the matter, but at least you could pretend to his mother that you didn't do it before he was of adult age.