Is your husband working two jobs plus overtime? He souldn't have time to baby this adult if you are working that kind of schedule with two children to raise.
Remind your new husband how much he is actually hurting his son by not teaching him the respondsabilites of adulthood. Also if after talking to him about how you feel he cannot see the financial harm he is doing open a bank account with just your name on it, when he leaves to pick his son. Transfer all the money. He will be angery but it will force him to pick up the phone and talk to you. Talk until he undestands your point of view then deposit enough back for the two of them to get home. When they get there son gets a job to get him back to school. The money only goes back to the joint account when the son has accomplished this or your new husband proves his priorities are in order.
You may also share with his this...My mother is 58, in forclosure, has no retirment savings, no life insurance, no health insurace, and cannot repair the hole in the side of her house. My 20yr old sister is a college student with a full scholarship, paid for apartment, utilities, groceries, gas.She works one to two days a week and still expects our mother to pay for car repairs, clean-up after her, do her laundry, buy her new cell phones when she gets them wet, pay for her romming charges, and pay for her to go out with friends when she returns home every other weekend.
My mother has created a monster and in her poor state of health she is at very high risk for a heart attack. What do you think will happen to my sister if the worst were to happen? Her grades would fall under the sudden and overwhelming respondsabilty causing her to loose her scholarship, She is likely to ruin a lot of clothes which she won't be able to afford to replace and will probably eat nothing but cereal, she will soon find herself with out a car or a phone. Stranded up at school with no apartment when the semester ends, no education, no communication, no confidence, and completly lost with out my mother holding her hand.
This is not a situation we place people in when we love them. As parents we are there to help them up when they fall off their bike, we teach them how to ride so they don't fall down, but we do not run behind the bike holding on forever.
Good Luck!