My ex is signing on my son's checking account

Worst limerick ever.
But seriously, contact the bank as soon as possible, and a lawyer immediately after.

Depends whether the ex is “in the process of bankruptcy” or simply went through bankruptcy court and uses the other name because nobody will deal with him personally. If it’s assets he’s hiding from bankruptcy or orderly repayment processes, that’s one thing. If it simply expedites his cheque-cashing and other money-handling, it may or may not be fraud.

Doesn’t an account for a minor need an adult to also co-sign or something when opening it (even if they don’t need to co-sign cheques?).

Regardless, tell him to find some other victim; if he’s screwed up his own finances, odds are he’ll destroy his son’s financial reputation eventually.

Not bad, Bryan Ekers; not bad at all!

“my son doesn’t even know what’s going on…”

There also must be a law somewhere about opening an account in someone else’s name without their consent, even if they are a minor and a dependent. (Assuming he is). I can see a savings account for a 2-year-old or something, but by the time someone is 10 or 14 you would think that opening an account without their consent or knowledge, and for your use not theirs, borders on fraud.

Wish the OP would check back in.