My husband has had full custody of his 17 yr old son. In the past yr my husband has had multiple surgerys, and has been advised by many doctors he will never work again. His ex wife is now wanting full custody of his son and want him to sign off on a paper that when he gets approved for Ssi that she gets 70% of his back pay. Can she do this?

Can she do what? Want custody and money? Of course she can.. want. However if your question is can she get it, that's a different matter. He does not have to sign anything he doesn't want to unless a court orders it.

If the son is 17, then by the time a court can hear this petition he will be close to 18 or over it. At which point custody becomes moot, he can live with whomever he wants. As for support, generally its based on an income based formula.