To assume he has stolen DS money?

[archlight]

My ex and I split when DS was 2 (he's now 15) His parents invested some money in an account and just recently remembered about it. I had remembered as I get the paperwork to me.
So the other week ExP comes to pick up DS15 and asks if I wouldn't mind signing something. I was a bit taken aback and signed it.
It looks like he's signing his parents investment into his own personal account. He claims he's been saving monthly for DS for at least 10 years (which I find hard to believe) and this is a savings account which had great interest,
I now think I've been stupid and I've actually just signed this money into his own current account.
Knowing his parents I think they'd be angry at this but how do I approach them without sounding like I'm accusing their son of stealing DS money?!
I know I shouldn't have signed anything without knowing what I was signing but DS was there and it was pretty awkward.

If you've had to sign something you must be one of the account holders. Ask the bank.

Ask the bank

It's not a bank account, it's an investment thing. Is it not too late if I've already (stupidly) signed for it?

As your signature was needed you should be able to get a statement.

Is he the type of person who would steal from his own son?

If you signed 'the other week', it's too late to rectify the situation, if that was indeed what you signed. If you have the details of the investment, you could 'phone the investment company to find out what the current situation is, then at least you would know.

@Weenurse yes unfortunately
I feel really stupid

If the paperwork is sent to you then you must be a trustee or similar. I don't know what you signed but dig out the paperwork and call and find out. If they don't give you info then you know you've given up any links to the money.
If it's a lot, I'd ask ex's parents what age DS was to get the money. Your ex can't just steal from his son, if that's what he's done. He has duties as a trustee if he's got his mitts in your son's money.

I would speak to the parents. If he has stolen it, you may need to go to the police.
Do you know how much it was?