Child Support Payment

I have two boys that i receive child support for. my 18 yr old left home on his own this past September and moved in with his father. my 16 yr old still resides with me. their father stopped paying last July and currently owes me over 3thous in back support.

He is now taking me to court for child support for my 18 yr old who is still in school. he said i was suppose to report the change, but i was told by a lawer advice its his obligation not mine to file.

What am i to expect when i go to court? will they make me pay even though my son left on his own? who was responsible to file for a change?