That's certainly a tough situation to be in, and, unfortunately, not that uncommon. It can certainly take kids time to adjust to the thought of their mom or dad dating someone new. So, I'd definitely take it slow as you ease into his life and give him time to get used to this. Also, I've read that it can be a good idea for the parent-your girlfriend-to really sit down and talk to her son and make sure he knows her love and commitment to him. Tell him that she realizes no one could ever replace his dad. Sometimes kids are just struggling with insecurity in situations like this. Lastly, I think there's a lot of good tips on things like this in Ron Deal's book, The Smart Stepfamily. I also believe he has a website with a lot of articles that might be helpful. So, just FYI...