As much as I'd love to provide you with a world of insight, unfortunately I cannot. I can only tell you of my own experience, and perhaps therefore I may have an attitude that is a little 'jaded' or 'persuaded' by that experience.
See, my now ex husband was also unwilling to discuss money at length. He too would often answer "I don't know" or "Don't worry about it, I've got this." ...... but wouldn't actually give me details of what it was that he 'had', presumably in his mind, he had it 'under control'.
He would take out loans on top of the mortgage (and at first I agreed to these loans, but not realise myself the effect it would have on the mortgage), in order to 'work on his cars' ....... and things slowly began to spiral out of control.
I then became unemployed, at which point he began lying to me about money, saying he had none (when I asked for money for groceries or goods that we needed) but then would come home with some car part, for his club car, not the main car, that was worth hundreds. And I think too, in the whole decade plus that we were married and/or together before marriage, I think I only ever saw two pay slips. He kept those hidden.
I could tell you more about what happened, but basically we are divorced, as I said at the start; he is my ex husband, and money issues was certainly one of the main problems in our relationship. I had actually threatened to leave one time a couple of years before I left because of the financial abuse and the lies I had exposed.
Money management is a very important issue when considering moving forward in a relationship, and I would say that if you are having doubts now, then perhaps consider not moving forward any further, until or unless you can both REALLY be on the same page about money; open and transparent.
To give him the benefit of doubt, maybe he just doesn't know how to talk about money? Such as when you did the calculations without him knowing, maybe it would be better to do those calculations with him? Perhaps maths isn't his strong point, but he may be embarrassed about it?
Anyway, I don't know if that helps at all. I hope it does help at least a little. Like I said, try doing some calculations WITH him, and see how that goes.
Take care. I wish you all the best in sorting this issue out. xo