Rock and Run still have some tiny Five Ten Rogue VCS shoes in adult size 1 - 2.5 (more like 12, 1, 2) £20 a pop - amazing quality. I got a load for my local wall to use. Kids love them.
I know we all have different financial situations, but do you really want to save a few quid by putting your son in a used harness with unknown history? When you can get a new kid's harness pretty cheaply anyway?
.... I think there's a basic Simond child's sit harness at Decathlon for £20, IIRC.