If that were the case, woofgang, then surely that would give people carte blanche to leave behind any old rubbish they couldn’t be bothered to dispose of.
I moved at the end of September, and on one of the forms, there was a section where I had to agree to remove all items, unless it was previously agreed they would be left.
Assuming the property was bought, then I would ask their agents to contact them, and give them say 2 weeks to arrange a convenient time to come back, but if they fail to do so, any costs incurred by your son to dispose of them, would be pursued through The Small Claims Court.
I would also back this up with a recorded delivery letter. Your son could address it to his address, but if the previous owners are having their post redirected, it should reach them.
If he gets a knock from the postie one morning, though, it didn’t work!