The boarding pass of your BIL. will need to be scanned for check-in in order to hold the seat. If it's not scanned, they will not hold it if there are standby passengers. If you ask them, they will tell you that it can't be done.
Whetheryou choose to present the boarding pass as if it were the child's is up to you, probably won't be noticed, but is definitely against the contract of carriage, and might be illegal.