His wages will stop anyway as from teh 17th- at least he won't accrue any more (unless there is oustanding holidays) but if he's normally paid at the end of the monthI'd expect them to leave his final payment until the payroll is run. As for witholding his p45 I am not sure that would be a problem- it's only to help sort out the tax aspects.
When I change jobs I usually surprise myself at working hard right to the end- it's in my nature and probably in your son's nature, to do the best job he can, but he needs to set boundaries- e.g. make sure he leaves at 6pm each night.
As for DO NON LET HIM leave on the 17th, they can't unless he didn't give the required notice, but even if he was due to leave later there is not much (there may be nothing) they could do about it