You don't indicate where the step-son lives or which state issued the support order. If the support order is from Pa and based on Pa law then the child support obligation terminates at age 18 or upon graduation from high school, whichever occurs last. The obligation for child support can be extended by agreement of the parties or other circumstances (ie. the child has a disability). In Pa there is no mandatory obligation to pay for college tuition or expenses; however, this, too, can be established by agreement of the parties.