S.S.
1) I would ask, did you get everything in writing when you agreed upon amounts? maybe even supporting documentation, such as emails. 2) I would find a lawyer in her state and where your step son currently resides? Each state has different rules about back child support. 3) Can you prove you made payments to his ex? Not just say, oh I gave cash but canceled checks, your bank statements to show transfer of funds, etc?
In my two cents, if he's not going to trade school or college, your husband's support ends at 18. Sounds like your son wants extra allowance. Also, your husband may have to pay his favor share (according the the courts) and it may be more than your budget will allow.
Good luck,
S.