Why three meals per day? If you're not an expert, then can you point to experts saying that eating just once or twice per day is "chronic stress" and a "bad idea"?
The onus would be on those making claims. It's for him to support the claim that one a day is healthy, but you are conversely making unsubstantiated claims about the health effects of just eating one meal a day, and the onus is on you to substantiate those.
It is one thing to ask him for evidence of his assertion, another to make your own and expect him to disprove them.