If they believe that his mental condition would improve if they could tube feed him, AND if he was in his right mind, that this is what he would want, then he might be sectioned and treated but practically, as its likely he would need to be sedated to be treated, it might not help, as Sqad has said. Chrissa my thoughts and prayers are with you both.
Psychiatrists and psychologists do give help with the management of people whose mental state is caused by an organic condition but would not treat them as such.