Tell him to get closer to Allah, and ask Him to cure him from his disease, and if he doesn’t get cured , he has to know that this is a test for his eman, if he passes this test in dunya he will pass in the hereafter.
Remind him of the hadeeth that thebone who looses his beloved eyes he will get the jannah.
And this is a promise for those who accept the choice of Allah. If he doesn’t pray, he is not a Muslim anymore and he is in great danger.