K.P.
There's a great book called "When Dinosaurs Divorce"- most local libraries carry this series. I would go get it and read it with him regularly. By talking about the dinosaur family, you can open a dialogue with him on his vocabulary level without discussing your situation (which is highly emotional). At the end of the story, talk with your son about the different characters and the emotions that they expressed. Then link it to your own emotions. Tell him that you love him and that you get sad when he's away and that you get very excited when it gets close to the time you will see him again. Keep repeating the same message and he will understand.
As for not taking advantage of you... keep the household rules consistent. He will test the limits b/c he's a kid and a new family structure may translate into "new rules" in his mind. Depending on your relationship with his father, I would suggest having this conversation together with your son. He's little, so keep it short and to-the-point. He'll only take advantage if you let him!