Hello! We live in Texas my husband pays child support for his son that is 15 year old but his currently in jail and his 17 year old daughter she drop out of high school. Does he still needs to pay child support into they both turn 18?

This is usually dependent on how the support order is written. Most orders will state age 18 or until they finish high school whichever is LATER. So you need to read the order.

If his children are in that much trouble, maybe he should petition for custody to help them (which can also end his support obligation.

Also, How long is the 15 year old going to be incarcerated for ?

Until the court order is changes he still owes support. He can petition the court to modify the support order. He does need to know a couple things before doing so. One being already asked, how long will the 15 year old be incarcerated, and the second how soon the 17 year old will be 18. I say this because it is common for it to take a couple months to get a court date. But the bottom line is that while the order is still in place he still owes support. Having an order from the court would mean nothing if the person that is supposed to comply could just vacate the order whenever they decided it was time.