[Solved] Feeling used and disrespected

Sorry for the long post. But advice needed. I am feeling like my step son is just using us. He is nearly 13. I have been his step mum since he was 6. My husband doesn't have a good relationship with his ex wife, she has always been desperate to drive a wedge between him and their two children 13 and 10. We See them 1 weekend a month and school holidays.
10 year old is great and always has been. 13 year old is quite devious, and I think has been increasingly using us for a long time now. Arrives and starts asking my husband then me to buy him expensive stuff (he has always been materialistic) only wants to come in school holidays if we are going away on holiday or will take him to Alton towers. He seems happy enough when he is with us, and joins in well, gets on well with with our two children (3yrs and 11months)
However he increasingly seems to be telling his mum how he does like it here, telling her he wants to change his surname to the same as hers etc. I believe his type of behaviour will definitely be encouraged and at times rewarded! His sister told us his mum says that if he didn't come here for the weekend he could have £150 for a new watch (which happened). This is just one example of her buying him, which does work in terms of him not coming to see his dad.
We go 4 weeks at a time without seeing the children and my husband once a week with text his son, just a how's school, general hello type message, and it will deliver often during the school lunch break, but he never replies. When we ask why, he usually denies getting it. His sister has said that he often 'sucks up' to his mum and tells his mum untrue things to get things he wants or his own way (often negative things about my husband or me)
When my husband tries to discuss with his son if he's happy here, and if he still wants to come to our house he always is adamant he does and when asked why he is telling his mum he doesn't l, he just says nothing or just says sorry.
His sister doesn't do any of this, she is always grateful, happy, is quite open that she loves her mum and her dad.
Their mother has always concentrated on her son more. I am very aware that he is probably under a lot of pressure to be negative about his dad and us to his mum and I don't want to add to it, but I also don't want to be left feeling used and also I don't want to bring up a child who thinks that it is ok to treat people like that, or to tell lies to get things. We have the other 3 children to think about and we want to be fair to them all.
We had them for Christmas and despite much prompting he was not grateful (never said thank you once) for his new Nike Jordan trainers that he really wanted.
I know he is saying untrue and and unkind things about my husband to his mother and it does upset me as I don't want a child who thinks its ok to be disrespectful and dishonest.
We never compare the children to one another in front of them or so they can hear. My husband is very careful to always have discussions with both of them separately, even though his daughter seems fine, when he spoke to his son about if he was happy with us etc he then had the exact same conversation with his daughter, ensuring he doesn't treat them differently or as if one is more important than the other.
I don't know what to do, I feel angry and upset. It's his birthday next month and he is supposed to be with us and I imagine he will want to come for an hour collect all the presents he has been demanding (an expensive Segway!) then go back to his mums........what do I do? Put up with it? Try to speak to him and be the 'nasty step mum' ......