Tony first of all I'd like to commend you for being frank and open minded in posting your problem on this forum.
I agree with various people who said that why put a large jet in the coffin corner when FL370 was available. That definitely is wrong decision making. I also agree that both of you scored very low on basic CRM issues.
Coming to your point of how this Captain came up with his decision I wish to say the following. Many Captains who have accumulated substancial experience in one type of aircraft have the ability based on previous experience to know the ability of their aircraft at any time. Sounds weird but believe me it is very possible and it does not require genious. First of all the FMC is a guideline, the QRH is the limiting factor. Now how did this chap know, simple maybe 5 minutes ago while looking through the FMC he noticed the MAX as being lets say 409. I agree that when questioned by you as to how he came to his decision he did not elaborate but you must admit that your attitude did not encourage discussion either.
All in all it's good to question things that make you unhappy or un-easy, however I do not think that this was a reportable event, I believe you could have discussed it at the hotel over a few beers.
By the way it sounded from your description that this Captains attitude was known to you, had you had any problem with him in the past?