He began the grooming when you were 15.
His parents have no right of abode in the UK, they can visit the UK but cannot stay for a period longer than 6 months.
You need to complete your education, he needs to find a job.
He’s all you have ever known, so he has a monopoly on your feelings, you should openly entertain the prospect of meeting others and see what they bring to the table before you accept your fate.
Some of the pitfalls of online relationships are, its easy to mislead and the fantasy rarely matches up to the reality.
A few more face to face visits are called for (Stoppit and Lipstick summed it up nicely).