In BC, if my child is turning 19 on Sept 1, and moving out…

When you say your son is moving out, do you mean permanently or just that he is going away to school?

Will he be living at home with his mother on the holidays and during the summer etc?

If he moves out permanently then he is no longer entitled to support from either of you.

If he is just moving out to go to school then he is entitled to support.

But the amount you would pay his mother for monthly support would be reduced, generally to 1/3 of the amount you've been paying to reflect the fact that he is away from home for 8 months of the year.

The 1/3 would still go to her as it is for the months he is home with her.

You both would contribute to his school expenses and for that you could give your portion to him directly and she must give hers.

If he moved out permanently then you obligation to support him is over,.

If he moves out permanently he is actually not entitled to any support which would include the tuition.

He is no longer a child of the marriage when he moves out permanently.

You would bring a motion to Court to enforce the Agreement.

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If the mother agrees you can do that on your own.\

If not you may need to get a Court Order and you can self-represent if you like.

There is no set way but essentially you will pay for 2 1/3 kids.

So your method makes the most sense and it's how I would do it but there is no guarantee that a Court would do it exactly that way.

It's for sure well into the right range.