Partners 9yr old son

[ed2712]

Im currently in a relationship with a lovely great mother of two great kids, and we are hoping to get married next year, but her son is making it incredibly difficult i love him to bits and he says he likes me, but as soon as his mother is out of the room all hell breaks out, unless he has my 24/7 attention its getting me down and i fear my relationship with his mother is deteriorating. His little sister is being pushed out to the side but if asked she is quiet happy to go play but her brother is so insistent i cant even go to the toilet without 20 questions.
PLEASE is there anyone out there how can give me some advice on how to cope with this as its starting to get me down.