This.Talk to someone from Legal Aid about your rights.
I'd say if he refuses to move it, you're entitled to call the scrap metal or wreckers who will come get it for free and might even give you some cash.
You'll prob need to send your nephew a letter giving him 14 days to remove it before you take action. Talk to Legal Aid on the phone first.
If someone dumps something on your property and refuses to move it, it will become your property after a certain time (and not doubt there's a process to go through too - letters etc). Legal aid should point you in the right direction.