Aw...he sounds like a very imaginative little guy!!
I don't know if this would be something that you want to give for a Christmas gift, but just as an activity in general...
Buy a large, clear, rectangular storage box...not too deep. The kind they sell for sweaters to store under the bed. Add about an inch or so (you don't need a lot...believe me!) of soil, sand, dried coffee grounds, or make "clean mud"
http://www.earthskids.com/sensoryrecipes.aspx#mud .
Add some cars, trucks, small cups and shovels, pvc tube, etc.
Spread a large flannel backed plastic table cloth on the floor (buy at the dollar store or at any store after Christmas for CHEAP!) and either put the box right on the table cloth or put it on a low table and he can stand to play.
When he is done, just put the lid back on the storage box. You can put other things in the "sensory table" as well....macaroni, dried corn, shredded paper, dried beans, water, corn starch mixed with water (ooblick...very cool stuff! It has the properties of both a liquid and a solid at the same time!)
The possibilities are endless!! It does require some supervision, but he will LOVE it!!
(I'm a preschool teacher, and we do these things with our two year olds. I will tell you that they learn the rules VERY quickly. Whatever is in the sensory table MUST stay in the sensory table. If you spill, you stop and pick it up. We use gentle hands...no throwing!)