We have taken our daughter each year around her birthday. She will be turning three in August and will be making her third trip to DW. While she was younger than your son is now it's not so much the memories he will remember or you will try to make but the ones you will have. For instance no matter how special we tried to make the trips and tried to make memories for her the best memory is not one we planned or one that she remembers, however, my wife and I do. We just so happened to be in DHS trying to kill time before our reservation at the Brown Derby when we went to the animator's studio to see what the waits were like for characters. The only charachter out was Mickey and the wait was insane so we decided to leave. As were leaving we saw Jake had just come out but appeared to have a long line as well and we could not afford to wait as we would risk losing our ADR. A cast member informed us, however, the line was not for Jake and that Jake was all alone with no wait. Jake proceeded to dance with my daughter for approximately five minutes. Then out of the door behind my daughter appeared Pluto and Minnie, who spontaneously joined in dancing behind her. When she turned and saw the two her face lit up and it was like she had her own Disney Character party if only for a little while. My daughter likely doesn't remember this moment but her mother and I got fantastic pictures and video. It is a memory my wife and I will tell my daughter about for years to come. Though my daughter constantly reminds us how much she loves riding IASW, she is even at the point of having to listen to the song constantly in our car.
One final note, it is has been interesting from year to year to watch how my daughter reacts to different attractions, characters and the like that she saw the year before. We cannot wait to see how she reacts this year, esp. since she constantly asks to watch the videos of her last two trips.