9 yr old switches behaviour on and off

[Bex593]

My 9 year old DS's behaviour has been getting progressively worse. For me and DH, he is argumentative, uncooperative, grumpy, nasty, starts arguments and is full of attitude etc etc.
I split up from DS's dad five years ago. His dad remarried and has another child. He sees his dad regularly and we all get along.
I've always maintained DS is in control of his behaviour and is able to switch it on and off. This has never been more apparant than recently. I was having a barny with him and his dad walked in. He immediately changed his behaviour and was nice as pie. This has also happened with other family members.
DH is getting to the end of his tether and has asked if I want him to leave, as he feels unwelcome by DS at the minute. I just feel drained and exhausted and hate to admit it, but dread spending time with DS.
We've talked to DS a lot about how unacceptable his behaviour is and asked him why its just us that gets the brunt of it. We've tried sticker charts and other incentives, but nothing seems to work.
Any advice greatly appreciated......