Versions of Vegas Pro prior to the current release (which is Vegas Pro 22) are no longer available for sale from MAGIX and thus Vegas Pro 15 hasn't been available from MAGIX since the release of Vegas Pro 16 nearly 7 years ago.

You'd need to be looking for a private sale of Vegas Pro 15 on websites such as Amazon, eBay, etc, etc. But be very careful because it is not unknown for scammers to sell discontinued software but they sell the same serial number to that software multiple times and the scam victim can't use that serial number because it has already been registered by someone else.

Is there any particular reason why Vegas Pro 15? Your son might want to a free trial version of Vegas Pro 22 - projects created in earlier versions of Vegas Pro should open successfully in Vegas Pro 22.