SS moved out, what are we responsible for?

Background: Husband and Bio-mom shared joint custody with equal time until 3 years ago when their mother moved out of the area (still in the state). The youngest ss moved with her and the oldest stayed with us. Though legally he did not have to, my husband kept paying the same amount in child support that he paid for both $500 a month. He was there from June 2012 to January 2013 and then moved back in with us. At that time, the custody was changed so that my husband had primary custody of both boys, and no child support would be paid by either parent to the other, At the time was 17 and would soon be 18, graduating high school, and going into the Navy. The youngest was with us for a year and 1/2 and then in July 2014 at 16, he decided to move back to his mother’s. No paperwork was filed or changed at that time. They made a verbal agreement on $250 a month he would send her to help out.
Since he’s moved to his mother’s, he’s had 3 tickets, one for speeding, one for reckless driving, and one for failure to complete motion…I think he was given a break on not stopping while at the beach, had his car impounded, and had 3 accidents. He was also tardy from school 18 days last school year because no one was there to wake him up.
He began talking to us over the summer about moving back in with us so he could have his Senior year here at the high school everyone else in the family graduated from. I should mention that he’s bipolar, though they won’t diagnose until he’s 18 and he’s been on medication. There’s been a [b]lot[b] of drama in the past between his mother and I, between him and his mother, between his mother and my husband…basically his mother and everyone involved.
Last week he came to visit and once again began talking about moving back in with us. After discussing it, he’s not given a good reason other than a job he possibly could get that’s part time. My husband and their mother discussed it and it was decided that he would finish school where he is now and then move back if he wanted. He’d be 18 then. He asked about being emancipated and what would have to happen. He even asked about “leaving” but I think essentially he meant running away.
Friday, it was decided that he would be moving back in with us and had a job interview today. He was supposed to come up yesterday, Sun. Yesterday morning his mother sends a text message to my husband saying essentially that he’s already moved into his new home with a friend and his parents, in our town, and that he’s of age (17) and this way we wouldn’t have to deal with him anymore. That they love him like he’s one of their own, and it would be nice if we met the couple that he’s moved in with.
Needless to say, we feel as though we’ve been sucker punched. The pictures on Facebook show he was here on Sat, already moved in apparently, so the sneaking around and planning has been going on for a little while at least. We have never met them and up until yesterday didn’t know their last name. I only figured that out through Facebook.
We do not plan on forcing him to move in with us. If this is what he wants to do, then we just want to know where we stand. Morally we still feel responsible for helping him with school supplies and clothes, but I think that feeling of obligation may go away after he turns 18 in April.
My husband is going to see an attorney today but I wondered if anyone else has been through this?
Legally, financially what are we responsible for? Has their mother, or this couple broken the law? I don’t believe my husband is still obligated to pay their mother, since he’s no longer living with her, but would he have to pay the child support they agreed on verbally to this couple?

The court doesn’t recognize a verbal agreement on child support, so as it stands that $250 obligation means nothing from a legal standpoint, and he can stop at any time. The mother can only enforce a valid written agreement or court order on child support.