YABU. You have to make this a priority and find a way. You cannot let him start high school with no uniform. It's cruel, he will get into trouble, and the other children will always remember it.
He will be nervous enough starting, don't let him stand out like this as well.
There has to be something you can cut back on, or something you can sell, to get him the bare minimum. Even if you download a few survey apps on your phone, you will probably be able to save up a decent amount for the uniform over the next month.
Buy the absolute minimum and build it up. A cheap set of shirts, one pair of trousers, a tie, a blazer.. the least amount possible you can get away with for his PE kit.
Find parent groups online, or even local community groups, and put a post on explaining your situation and ask whether anyone has uniform their child has grown out of.
But don't let him go to school without it. Beg, borrow, steal before you do that.