S.T.
Hello, I think everyone is on the right track with the back pack Idea. My daughter has been carrying her own toys, and her own diapers and pullups for a long time. She loves it because she thinks she is like Dora. If he likes Diego or any other character that carries a back pack you should explain it to him useing that. It will take getting used to...but he will feel like a big boy that way.
Hope it works!