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No, you do not have to pay while he is in college - as you stated your obligation ends when he turns 18 and/or graduates from high school. If, as you stated, there is nothing in your marriage settlement agreement to the contrary.
However, you must petition the court for the child support to terminate upon his reaching 18 or graduating - the child support won't automatically stop but will continue until you petition.
How do I go about petitioning for termination of child support?