My experience is that the efforts an average 6-year old should put to learn programming at that level are very high and at ~9 years old not so high. So except he is very interested/capable I would wait until he grows. I don't think you gain too much starting earlier, in a way I call it parenting anxiety. This is why you don't start teaching them calculus, they are not ready except if they show signals of precocity or giftedness.

Having said that I would start with ScratchJr (https://www.scratchjr.org/) on a tablet and Alice (https://www.alice.org/) on the desktop. Alice will require your assistance for scripting but he would be able to select and position the 3D characters.