update: I've looked after my nephew now for a few full days and taken your advice - worked hard to try and build self esteem. Did lots of fun activities, praised the good things that he'd done and when he tried to create an issue so as to go into a tantrum I distracted him. Exhausting especially given how little he wanted to contribute.
Sadly the one overriding factor was how aggressive he was. He'd scream, throw himself around, hit himself hard frequently. When playing near other children he'd move towards them and then try and knock them over. The other children were always much smaller than him - he never tried it with kids his own size. We fed animals and then behind my back he threw stones at them. As a little girl came down the slide he threw sand in her face.
These were all things he did whilst enjoying a nice day. The grandparents quickly got defensive when his behaviour was drawn to their attention. In fact they came out with some BS that it had just started in the last week - but clearly he's been a bully for a long time.
I won't give up on him but I'm left in a bad mood after the time I've spent with him. Not because he's so destructive, unhelpful, ungrateful, negative, exhausting - all of which are a result of how he's been brought up, but ultimately because I don't feel his parents really care or his grandparents care. In fact I get in trouble for pointing out what he's like - they think I'm negative and got it in for him??!!
Should I be making an issue with the grandparents about what a bad example he must be as a JW kid? Is that the only way I can get through to them? (I don't want to bring up the JW thing if I can help it - no knowing what that'll unleash).