I am so sorry you are feeling this afraid and lacking in trust in your partner.
Children say all sorts,and I don’t know your child, so I can’t judge on that. I also wouldn’t ask them more, because information might go both ways
More pragmatically, if you are at all in doubt about your relationship, and unless you are 100% financially in control - which sadly comes first - and secondly have emotional stability that is strong enough in relation to your partner to know when and how they might be stepping outside of your agreed terms and what the consequences are if they don’t - do not, EVER, show your cards.
I cannot emphasise the power of being ahead enough.
Whatever the truth is, you may never fully know, but you will have to deal with whatever comes out in the wash eventually.
If you can provide for yourself and your child it will be so much easier, so don’t give him a heads up you feel something is off any further, unless it is beneficial to you.
If you have independent means, or you can live within what the system may provide, you can risk it. If not, weigh it up.
Prepare now, while you have time, for the worst and hope for the best.
I am a never-share-a-bank-account sort of person so I am biased.
I am also of the opinion that if a woman engages in flirting that, if it was done by a man, would be considered creepy, and those you trust with tour well-being don’t recognise that, something is off.
Long story short - sort out the money before you rock the so far, non violent and not emotionally unstable boat.