While I've never been in your situation, I've had a lot of experience with kids (babysitting, working in a daycare, and coming from a large immediate and extended family). You know the little boy likes you already, so do your best to relax.
Here are some things you can do to help make things go a little smoother and maybe help you be a little more relaxed with your boyfriend's son... Get interested in his interests. For example, my 3 year old loves Transformers and Teenage Mutant Ninja Turtles. Ask your boyfriend what his son likes. Perhaps he likes a certain game or sport. Learn about it and then use your new knowledge when you are talking to him. For instance, we ask my son which Transformer is his favorite. He tells us his favorite is "Meets The Eye... The one from the Transformers song."
Find out what kinds of things he likes to do. Things you can do together like playing a board game, playing minigolf, going to a video arcade, or playing laser tag. Then during your vacation, plan an activity that you know he likes to do.
Simply ask him how he is doing and what is new in his life. One of the easiest ways to get to know him better and build a better relationship with him is to talk to him. Take a moment when the other kids are busy and he isn't and just talk to him about everyday mundane things. How is school? Is he signing up for any Spring sports? Is he on the Chess Team (my daughter is into Chess at her school)? Talk to him about what is going on in his life and give him a chance to share and open up. Ask him about his friends and what they like to do.
While you are doing this, remember that you are the adult but you are not his Mom or Stepmom (yet) so take this time to build an open, honest relationship without the bossiness or the need to be the disciplinarian but do not become so much his friend that you allow him to disrespect you.
Anyhow, I know I haven't been in your situation exactly but having needed to get to know and build a relationship with kids (both easy and difficult kids) in the past, I hope that my experience and advice helps you out. The most important thing to remember is to relax and let it happen.
Good luck.