H.G.
I've been through this exact situation. My son was almost 3 when his little sister was born. We prepared him the same way you did with your son. When the baby came he was totally in love with her, but very aggressive with us. It just takes time for them to adjust. What helped my son was spending one on one time with me (without the baby) and letting him actively contribute to taking care of his sister when possible. I also talked with him a lot about how he was feeling. I felt it was important for him to communicate how he was feeling and understand that all those feelings about his sister were okay. Feeling, happy, sad, or mad even at the same time is normal, but throwing tantrums, disobeying, etc is not. My kids are 4 and 18mos now and everything is fine now. They don't always get along (what brother and sister do?) but they definitely love eachother.