get ready for one crazy and wild ride.
I was the stepchild, I have been the stepmom for 13 years so far and step-grandma for 6 years...
1. no matter what happens, the two of you must present a united front to the child at all times when the child's around... once they find a weak spot as they get older, they may try to play both ends against the middle.
2. your SO needs to make it clear that you have just as much right to enforce the rules and punishments as he does
3. I wish i still had it, but i read an article YEARS ago that helped with my role as stepmom that pointed out our "role" is more like that of a well meaning aunt.....
4. enjoy it
While my stepsons are all grown up and out on their own now, the one who lived with us until recently (he moved in when he was a teenager... his mother had no control over any of the kids and he used it to his advantage to turn the courts against her so he could get what he wanted, which was moving in with us LOL) never told his friends that i was his stepmother.... they always thought that I was his mom until we met in person and then he had to explain his mother lives over an hour away and i'm his stepmom
5. Don't refer to him as your stepkid.... adopt the attitude that the only "steps" in the house are the ones that take you from the first floor to the second... I always treated my dh's kids the same as the ones he and i have together and as a result ahve a great relationship with 2 out of 3 of them (the third no one sees, he's 18 and working at his first year of college).... as well as the oldest one's wife and 2 little girls....
6. it's going to be stressful and there'll be days you're ready to say bugger it and throw in the towel, but remember: You only need to deal with the ex and family courts until the kidlet's 18.
7. There are also MANY MORE good days than bad ones..... try to forget the bad and focus on the good.