I don't think that you should be the one to speak to him Tuzz - you are a threat to him at the moment. His dad should speak to him, but like most men, I would imagine that he is not good at emotional stuff and will avoid it all costs! If your partner cannot talk to him, then maybe ask a granny or an uncle he really admires to speak to him - not about his behaviour, but about coming to terms with the loss of his mommy and about coping on a day to day basis.
I think the best way to go about everything is just to 'be' there. Be kind, but firm, understanding, but don't allow him to be manipulative.
Maybe find out what his favourite computer game is and put it next to his computer without him seeing - he will ask his dad if he had got it for him and dad will say no - he will then know that it came from you and even though he probably won't thank you for it, it will show him that you have an interest in him, without making a big fuss (boys hate fuss!).
By now, you probably know what his favourite meals are - treat him every now and then by making it.
Find out what the latest craze is in pre-teen fasion wear - sneak it into his cupboard.
It doesn't have to all be about material things - arrange with your partner to have one or 2 photos of his late partner framed - put one in the boy's room and one in the family room - she is not a threat to you, she is gone.
When he wants to go to friends, quietly offer to take him - let him play the music very loud in the car.
Little things like that.