The reality is that, unless he seeks help or comes to the attention of 'the authorities' because he has displayed aberrant behaviour outside of his home, there is very little you can do other than encourage him to make an appointment with his GP with a view to having his physical health checked while, in the meantime, having written/spoken to the GP in question and alerted her/him to your concerns about his mental health.
It sounds as if he's taking practices from a number of eastern religions and cobbling them together to create his own path to spiritual enlightenment; in many religions white is the colour of purity, it's also the Buddhist colour of mourning and Sikh yogis wear all white in the belief that it expands their auras and enables them to practice mindfulness. Does he cover his head when he meditates or at other times?
The copper cup, together with a restricted diet, suggests that he may have an interest in Ayurveda which he is taking to extreme. Does he buy and prepare food for yourself and dd which falls outside of that which he chooses to eat himself?
Does he dress all in white to perform his new job in support work? If so, I can't imagine that this will go unremarked by his employers/clients and he may come to grief if he tries to persuade others to follow his current doctrine.
As he's not a member of a cult, the possibility of debriefing/deprogramming is neither relevant or applicable and I suspect that the stresses you've mentioned and, in particular, the sibling bereavement, have caused his planet to spin out of orbit, so to speak, and he has found comfort in pursuing what he considers to be the way to moksha.
What can happen in these cases is that a momentary lapse such as partaking of whatever has been forbidden - alcohol, a cream bun, a fag - may become the catalyst for more lapses leading to reappraisal on the part of the devotee and their subsequent return to planet earth what can be considered to be normality.
On the other hand, he may adopt even more unusual or bizarre practices and/or insist that you and your dc follow his example, take himself off to a distant land to sit at the feet of a master, or acquire a group of guilible followers who sit at his feet.
It's a great shame that he didn't go to a spirtualist church to develop whatever abilities he has a 'spiritual healer' as he'd still be able to enjoy a pie and pint and his newfound beliefs wouldn't impact unduly on family life.
Unfotunately, I can't see any way forward for you other than to make his practices a deal breaker, which you're best advised to do after your impending new arrival has been welcomed to, and is settled in, the world.