My son is a day pupil in special school but would benefit from residential - any ideas on how to pursuade the LA to fund this

I fought for a year to get my 14 son a day place at a special school which has made an amazing difference to him and our family wellbeing generally.  We now feel he would benefit from a residential place at the school which he would prefer.  It would help him with independence and challenge his OCD.  His annual review is due in June and I want to present a case.  Has anyone any experience of this?  I would welcome suggestions.