What a frustrating situation for you and your family! You are not alone! I've worked with several other children in child care settings that were also very curious about this substance coming out of their body.
Just a couple of things to keep in mind:
1:Toddlers are sensory learners. They enjoy playing with different textures. This is why play-dough is so popular. Try to re-direct this behavior with some new and fun sensory experiences. ex: make play-dough together (there are lots or recipes online), water play with food coloring or bubbles, shaving cream, plant something together and let him play with the dirt, make silly putty(1/2 liquid starch and 1/2 Elmer’s glue, goop (1/2 corn starch 1/2 water)...
2: There are 3 signs to potty readiness. 1) has wet/soiled diaper 2) is wetting/soiling diaper and 3) will wet/soil diaper. It sounds like your son is in the first stage and almost to the 2nd stage. A fast and easy read is Toilet Learning by Alison Mack, it goes into history of toilet learning, how to prepare yourself and your child then finishes with a picture section for the child. It's from 1978, but the information is great.
Best wishes! This soon will pass.