Unless the court action involves child custody and access, or enforcement of an existing child support order, than your ex has 'exclusive jurisdiction' as he filed first.
Now, ofcourse a Judge can use his/her discretion on choosing the location if it is
substantially more convenient to proceed in one place over the other.
As your child is an 'adult' and only lives with you part time, I doubt that you would meet the "substantially more convenient" threshold.
Unless you have a huge issue with travelling, I wouldn't bother fighting this.... it's an awfully expensive argument. 
Under the Family Law Rules - Rule #5....
IF the case deals with child custody and/or access, than the case is supposed to be heard in the same jurisdiction as the childs' residence.
Rule 5
(1)(b) If the case deals with custody of or access to a child, in the municipality where the child ordinarily resides,
IF the case deals with enforcement of an Order for support, then the case is to be heard in the jurisdiction where the support receipient...
(5) All steps in enforcement of a payment order, including a motion to suspend a support deduction order, shall take place,
(a) in the municipality where the recipient resides.
TRANSFER TO ANOTHER MUNICIPALITY
(8) If it is substantially more convenient to deal with a case or any step in the case in another municipality, the court may, on motion, order that the case or step be transferred there
Under the Family Law Act...
All proceedings in one court
(2) Except as this Act provides otherwise, no person who is a party to an application under this Act shall make another application under this Act to another court, but the court may order that the proceeding be transferred to a court having other jurisdiction where, in the first court’s opinion, the other court is more appropriate to determine the matters in issue that should be determined at the same time. R.S.O. 1990, c. F.3, s. 2 (2).
Under the Divorce Act....
Jurisdiction where two proceedings commenced on different days
(2) Where divorce proceedings between the same spouses are pending in two courts that would otherwise have jurisdiction under subsection (1) and were commenced on different days and the proceeding that was commenced first is not discontinued within thirty days after it was commenced, the court in which a divorce proceeding was commenced first has exclusive jurisdiction to hear and determine any divorce proceeding then pending between the spouses and the second divorce proceeding shall be deemed to be discontinued