Newish diagnosis - son not coping too well.

My youngest had no birth issues. He was apparently healthy but has always complained of mild leg pained has dyslexia. Although didn't have perfect co-ordination was much better than his brother. As he entered puberty and a growth spurt his legs became contracted. This worsened and last year, at 14, he was given a diagnosis of periventricular leukomalacia and cerebral palsy. Since then his legs have become worse and he now has lax toe joints causing dislocations and requiring orthotics to keep his toes in place. He also has developed tremors. His weight has dropped off with his BMI <0.4th centile (and would be less if he could stand up straight) He is under a neurologist and has referrals for gastroenterology, rheumatology, dietetics, genetics. So the truth is we don't know what is going on with him.

I'm looking for some advice to support him regarding low mood. He is refusing to engage with physiotherapy as he says 'why prolong the inevitable'. He has researched assisted suicide as he is convinced he is going to end up with locked in syndrome or MND. There is no evidence of this. Most of the time he is fine but when you sit down and talk to him he is overwhelmingly negative. 

Any ideas?