Ex-wife, son, grand parents

Hi,

I need some advice. It's a long winded one so I will sum it up in bullet points

1) I have a son with my ex-wife. He is 10. We spilt when he was 6 months. My parents have shown very little interest in him up until about 12 months ago. They made 3 trips to see him in 9 years and have openly informed me and my ex-wife how much they detest my ex-wife. I have taken him to visit them approximately 4 times a year. We lived a good 150 miles apart. They have another grand child which they lavish with trips abroad, private school fees, presents. He even has his own bedroom at their house. He was clearly treated very differently to my son which I wasn't happy about but because we were related I tried my best. I believe you should try and treat, or perceive to treat, all children/grandchildren equally. I cannot explain over email what they are like. It would take all day.
2) I have remarried. Up until my wife and I had our child, now 2.5years old, we had a good relationship with my parents (we jumped when they snapped their fingers) but as soon as our child arrived they became quite frankly, nasty and unhelpful to the point we decided it was better to not have anything to do with them. They then went on to bad mouth my wife, start making things up and as my wife found it difficult as a new mum to do the long distance trip. She tried her hardest to arrange dates to see them but all she offered was a "pop in for coffee" even though it was 150 miles to their house so impratical. She gave up and we cut off contact. Too many bad things were said by them.
3) We now have a second child together and my parents have seen our 2.5 year old for 30 mins and the 5 month old never. However, within the last 12 months my ex-wfe and mother, even though they have expressed they detest each other, have started seeing each other. My ex claims my son wants a relationship with his grandparents. My son says he's not bothered. We have found from my son he is interrogated by my mother as to what we are doing. My son is now lavished with theme park tickets, presents and even his half-siblings which aren't even related to my mother are getting the same. When he comes to visit us he's telling us all about it. I want to stop him seeing my parents as they weren't interested in him up to him being 9! Now since I have cut contact, they are all over him. I know they are doing this to get at me and my family and my ex-wife is so money orientated all she is thinking about is inheritance with no concern for our son and his emotions. I have explained to her I believe this is just a game and our son is a pawn in this game but she doesn't care. I can see him getting hurt when they get bored of this game they are playing. I also don't know how I am going to explain to my two younger children why my son gets this preferential treatment from his grandparents and they've never met them as they're not interested in them

What I want to know is based on the fact they have shown no interest in him up until the last 12 months and that they are quite hard, nasty people, that they don't give a [censored] about my two other kids, can I stop my ex-wife from seeing them with him? I have offered a compromise where I supervise the visits at a central location but my ex-wife has refused this.

Thanks.