Then the older child is entitled to be supported. Dad should be paying guideline support for 2 children, based on his income, and a share of the child's extraordinary expenses for schooling, usually paid in proportion to income or as agreed upon.
But it's never that simple. Often parents get into disputes over what should be paid and to whom at this age. So let's talk about that.
Child support is your ex's contribution towards the basics of providing a home for the child: food, clothes, shelter, transportation. The basics.
On top of that there are extras for school. Tuition, books, lab fees. Sometimes parents agree, or the court orders, that other things will be "extras" as well, like a car or cell phone or new laptop computer.
The most common cause for a dispute is the cost of the child living at school. If the father is paying guideline support, then he's already contributing towards shelter, food, etc.. So when a support recipient comes to me to say that the other parent doesn't want to pay their share of the child's apartment, or residence and meal plan (which is very expensive usually), I tell that parent that the support already covers that contribution. If the other parent is going to contribute towards the apartment and meal plan, then maybe the support itself to the support recipient ought to cease.
Items like tuition and associated registration costs paid directly to the school are clearly "extras", but the other items are a grey area. Sometimes parents agree that the payor will pay a share of everything and pay it directly to the child. Sometimes the support payor is ordered to pay a share of just the extras, and then falls into arrears so that the child and primary parents have to cover those costs and hope that the payor catches up. In short, it can be as messy or as clean as the parents want it to be. As long as both can agree on something that's fair (enough) or a judge makes an order.
Looking at your question, you asked whether you could be asking for more than his offered 100-150 per month for "housing and food". I take the view that these items are already covered by the support contribution, but that tuition and books and such are not covered.
So the best place to start is the exisiting court order or contract, see what it says about extraordinary expense contribution (sometimes called "section seven" expenses). Parents exchange income information. Support for two children is adjusted to the guideline amount if it's not there already. Then the parents agree on a list of what's an "extra" and what's not. Then the parents add that up for the year.
From there, you see what the child can reasonably contribute towards their own exta costs if any. Have they applied for all available scholarships and bursaries? Is there an advantageous position for student loans? Has the child saved anything from part time employment? The child doesn't have to put every cent towards school, but there should be a reasonable contribution.
When the child's contribution is taken off the total "extras" amount, then the parents split the leftover proportionate to the income.
Anything the parents can agree to as fair is probably fine, but it's always a shame to see parents who have got along well enough for a long time brought back to court on this issue.
I'm getting the impression that you two haven't exchanged income information in some time. You should. There's no point in arguing about numbers before you both have your starting point. So send your ex your last couple of years of Notices of Assessment from your Income Tax Return, have the child add up the school costs and talk to the child about what their reasonable contribution will be, talk about the need to budget and buy used books, about working early to find a summer sublet for the apartment, to look into what a bus pass will cost if needed or whether the child is going to be taking a vehicle, make up a basic budget.
Men are strongly driven by money. I'm sure that dad offered the extra 100-150 per month because that's what he's prepared to contribute at the moment. But whether that's fair depends on the costs and the math. So do a budget with the child and present the figures to the father. Then at least you both know what's on the table and you're not arguing about whether what he's offering is "fair", since fairness is based on what a court would probably order rather than what he feels like paying.
Does that make sense? I know that's a lot to read and digest, but this can be a complicated issue, and very much determined by the specific facts of the situation. If you want to clarify something with me then that's fine, please respond. If I've answered you then I'd appreciate a positive service rating please.