Accepting your stepson as your own?

I recently got married and it was the best day of my life...I've never found such a perfect women....the problem is she has a son that I never really liked or enjoyed being ard ound but always tolerated because I really enjoy being with her, even if I didn't get along with him as much...He himself is a good kid and smart but very bratty and seems like he's been spoiled quite a bit..he's also very demading, disrespecful and sometimes just takes things to far...I feel helpless because I can't show him the proper displine because he's not mine and he does see his actually father...overall the situation bothers me a great deal but I'm willing to work with it...Is there anyway I can make this transition easier? I've never have been more confused on what to do and it makes me furious...Again he's only 4 but he's very intelligent..and he spend time with his other family which is why I think he's so spoiled at times..It seems we have to cater to him 24/7