Hi!
That sounds exciting!
Are you sure it'll only be for the winter and he will be adopted prior to springtime?
Fosters don't usually have a set time that they will stay with their foster homes until someone adopts them, unless he's already called for.
It's always good to know so you can make sure you have enough supplies. 
Is he neutured and potty trained?
If I'm not mistaken, an XL dog kennel is only 48" by 24" or approx 8 sq feet, which is very small for a rabbit to be in for that long. The bare minimum often cited now is 16 sq feet, which is equivalent to the average dog xpen. I'd recommend getting a dog excersise pen instead: they're often cheaper and offer much more space. Xpens are ~$40 on chewy/amazon and just make sure to get one at least 36" tall so he won't just out! Regardless the enclosure size, if they're in one, they need IMO 8 hours or more outside of their pen each day.
What do you plan to use for the flooring? Is your room floor hardwood or carpet?
Of course only time will tell but if he's unfixed, young and/or a destrcutive bunny in general, you're going to want some suitable flooring. Rabbits don't have pads on their paws like dogs do, meaning that they just have a thin layer of fur over their bones which can often be painful and something called sore hocks can happen. Foam puzzle mats (like ones for kids or working out) with a flat sheet or fleece binder clipped on can work.
Check to see if you have any farms near you! Hay is often much cheaper through a farm than the pet store and if it just is for the winter, half a bale of timothy hay would work well.
A rabbit emergency kit is perhaps one of the most important things. In one, you should have infant gas drops (with simethicone), critical care, unflavoured pedialyte, syringes, exotic vet contact number, grooming supplies (combs, nail clippers), q tips and gauze.
Also, clear things up with the rescue you are fostering for. What supplies will the offer, and which do you need to buy yourself? Who will handle the adoption, you (or your parents) or them? Do they have a list of vets that the fosters must go to?
I highly recommend joining a rabbit forum such as rabbitsonline.net to speak with more rabbit owners to get their opinions and learn more.
Hopefully this helped and let me know if you have more questions!