once he is 18 there is virtually no chance of him coming to the UK unless he can get some work permit which these days is nearly impossible, at 18 under UK law he is no longer classed as a dependent of his mother.

him and his mother need to think now long term as time is running out.

also if he has contact with his father then that could be a problem, thou at 16 the embassy might get him in for an interview.