Wow Merlin, i feel sorry for you and wish you all the best, but a little bit of positivity wouldnt have gone a miss <:O)
My step-son can be a little s** just like any other child his age. We have all been together 5 years and luckily, he took to me like a duck to water - his Bio mother left him at 18 months and now he calls me Mum. So, yes, it can work!
There are days when his Dad takes his side and tells me its his duty, but thats because it is - regardless of whether im his bio mum or not! We all have our bad days regardless of who our blood relations.
ALC, get to know him, spend time with him and leave him to spend time with his dad, BUT do not allow him to walk all over you. Chat with your partner about rules etc - i did, and i made it plain that if i was going to be apart of their family, then it was now going to be OUR FAMILY and i would also make rules. We decided as a family what we all expected from each other and stuck to it. If my s/son exceeds his goals - i praise him, if he crosses the boundaries - i tell him off, thats how it is in 'normal' families isnt it?
P.S, he was 51/2 at the time, so age isnt the problem either- just make the conversation easy for him to understand
Good Luck
Flower xx