How does he earn a living if he needs to care for them 24/7?

Do you know what disease they have (I am not asking you to say it here). It is strange to be very very sick, incurable, yet they can live a long life *and* there is no carer but their son.

Thirdly, how you can help him? Leave him alone. He doesn't want your help. He cut you off and expressed it very clearly. He is an adult. He can make his own decisions. Clearly the 25 year friendship doesn't mean the same to him. So, move on.