In my experience people who hide their finances from a partner is down to two things.
- They have a lot more money than you think and want to keep it for themselves for whatever reason thats in their head. or
- They have a lot less money than you think and that may well include very heavy debt. In this scenario they are likely spending a lot of money on something they want to hide. Drugs , drinking , Gambling other people, sometimes bailing out family members or some other scenario.
The only way forward the is tenable for you , IMO is to have a 'drains up' conversation, this is best done again IMO in a neutral venue privately but in public. ( this can prevent tempers and emotions oiling over, and if they do boil over its better not to be in your house or with your child).
It is unlikely that trying to force your partner to comply to new ways will work, agreement is crucial and that agreement must be done with all relevant facts on the table.
I would think that this will take time and patience perhaps even setting out some sort of a timetable to get the thing done, again IMO its unlikely to be a once and done conversation.
Writing down your thoughts, issues, hopes and aspirations for yourself and the family will be helpful, that will help you keep your eye on what needs to be achieved rather than get drawn down rabbit holes which is very easy to do if you have no clear objectives. I would not share this at the start.
Have a really good idea of what you want out of your first discussion and i would suggest that this should be a realistic and achievable outcome. IE the next time we discuss this we need to have all aspects of our finances available and ready for joint discussion so that we can begin t agree a course of action.
It may be helpful after one or two discussions to get hime to write out his thoughts , issues , hopes and aspirations in the same manner.
If after all that you are in the same position , you will almost have no option but to go nuclear as it were,.
GL