My son has small bumps on his back, they're skin color. 2 weeks ago we thought it was from his pants. now it's all up his back. what could it be?

Umm, hard to say.: Skin rashes are most difficult to diagnose without actually examining them. Can you feel the rash? Does the rash itch? How old is son? Since your original theory that his pants triggered the rash seems no longer accurate, perhaps it would be wise to have your pediatrician or dermatologist examine this in person. Then specific treatment and preventive measures could be discussed with more accuracy.