Difficulties with Financial Planning?

My husband has ASD and I am seeking to understand more about difficulties with achieving a financial overview. He would like to send our children to independent schools but our income is not high and I keep explaining that we can’t afford it. He mentions other possible small future increases to our income and I try to point out that we still would be quite a long way off being able to afford it. He can’t see it, I find it’s like coming up against a brick wall and I’m trying to understand what’s happening. He doesn’t know any of the precise figures because he doesn’t like to engage with online banking or bill paying admin at all, so I’m in the process of putting it all in a spreadsheet for him. But I’m surprised even so that he has such an unrealistic grasp, as he knows what he earns, at least. He is very high functioning and does complex data analysis for his job, so it seems strange to me that this financial overview seems hard for him to achieve.