Some good advice from Scud… but… and this is something I get to deal with, but usually in older people.
Some of this depends on his age, and his mental capacity. People have the right to make bizarre and poor decisions, providing they have mental capacity and aren’t harming others. This can be very very difficult. Making choices like he has is not automatically a sign of mental illness, and even if it is, until the moment he lacks legal/mental capacity he gets to choose.
If he won’t engage with you/family/healthcare and you feel this is more than him being a bit odd, I would contact Social Services and perhaps say you think he may be a “vulnerable Adult” – the key words to get thinks moving…
Google “Diogenes Syndrome” for an extreme end of this sort of behaviour.