I'd contact the school or the band director. This seems extremely expensive. Is this all an activity fee, or does it include instrument rental? Check with local music stores to compare rates - they are always searching for used instruments that they can rent to kids.
Our schools have a flat activity fee per child (with a family cap) and it covers all activities. There may be instrument rentals and lessons for some things, but there must be a program in place to help families. Is this his first exposure to music? Then there should be something to encourage kids, get them involved, help them manage it.
By the same token, there has to be some commitment on the part of the child - that can be a commitment to practicing every day (usually 15-20 minutes tops for beginners), and a commitment to going to band practice (which is either early or after school). So set up some schedule for your child where he has to do something every single day for 20 minutes whether he wants to or not.
And what can he do to try to earn some of the money this summer? When my son was that age, he was mowing lawns, walking dogs, putting out trash cans and watering plants/gardens for people who were on vacation or on a day trip, raked out mulch beds, helped clean out attics when people were moving or doing yard sales, etc. He actually turned it into a growing business that he put on his "activity resume" for college applications. Have your son make up some flyers listing his skills and indicating that he's trying to raise money to join the band (rather than just buy video games) - I'll bet a lot of people will find that refreshing. Our son learned to bank that money (we had a formula for spending/saving and also giving a little bit to charity), as he got older he learned to invoice people and keep records, and so on. He had the keys to about a dozen homes in the neighborhood, and great references.
So make this a win-win situation - verify the costs, see what's available for scholarships, start now with helping him get motivated to earn money to help support himself and develop responsibilities, and then see what might be shifted around in your own budget to work together as a family to make things possible. It's a great experience for everyone.