My boyfriend of 2yrs has been sent to prison for 4mths for assault, it's the second time he's been inside. Whilst he has been in prison this time round, he has received a visit from immigration cause although he's been in england since 1986 (when he was 7), he hasn't got a uk passport (because of his criminal rec). Im so worried he will be deported, even though he has no ties, relations or memories of his birth country. Any one has any advice, Id be grateful.

That's not nearly enough information, and even if you provided a lot more, people online can't really know.
I have read news reports of increasing deportation in the US of people who were brought here illegally as babies, people who have no memory of their home and language. Many who have no criminal history. So I would be prepared for the eventuality, and hire a lawyer if you can afford one.

Are you in England ? Is that where he is in prison ? If so what is his native country ?

But immigration is getting tougher, esp for those that have committed criminal offenses