SS is 8. Lives with his BM full time is here two days (one night) a week.
Recently having increased problems, violence at home and at school,general backchat towards his mother. His hygiene is also less than to be desired, he ends up with stained underwear (both sides) every day, his feet stink and BM struggles to get him to have a bath/ change his clothes. He is very needy both here and at home, demanding constant attention and seems to struggle to play alone.
He is fine, very loving, and by the sound of it a completely different little boy here, goes in the bath when asked etc, we get a bit of cheek but that's about it.
The only thing we can put our finger on is BM has another child (a younger daughter) and he is maybe playing up to gain BM's attention, myself and his dad have no children, and when he is here he gets our undivided attention.

His dad and I suggested that he came to stay here for a few weeks in an attempt to give his mother a break, but BM was a bit unwilling to do this as she felt he would abuse it (i.e he would know he only have to kick up a bit of a fuss and he could come here for a few weeks and would see it as rewarding his behaviour) which is fair enough, I completely understand her sentiment.

We are going to see his teacher next week to have a chat about his behaviour (my suggestion) as we felt this would show a united front with his mum that we weren't going to tollerate this kind of behaviour from him.
Just to note we do not have a good relationship with BM, most communication is done through her mother as her and my partner usually end up at loggerheads. However nothing bad is said about BM in this house, we do nothing but support her (sat him down last night and said he couldnt behave like that with his mum). So it is difficult to sit down with her and discuss things as she often wont entertain the idea, or they end up in arguments, which obviously effects any solution.

So far we have sat and talked to him, repeatedly told BM's mother she only has to call if anythings a problem (BM and her mother as her mother looks after him after school), arranged to go see his teacher, I have bought him toilet wipes here, and obviously he has baths while here and gets changed often. Also talked about germs and smells as far as his hygiene is concerned. My partner did once threaten him that if he didnt behave that he would not come here (I was against this as I dont feel it will help with him seeing us as a reward). Also constantly reassuring him he is loved as I feel a lot of the needy behaviour is insecurity.
If anyone has experience of behaviour like this, or feels they could offer me some advice i'd be very greatful. Me and SS are very very close and I just want to help him.