My 3 year old son has bumpy skin on his back and bum its a little rashy on his bum but not his back. and he doesn't complain about it being itchy ?

Eczema: This is likely a type of eczema. Try hydrating lotion to the affected areas and see if that makes a difference. Of course, having a physician eye-ball the rash would be best.

?Keratosis Pilaris: If this is not itchy it could be keratosis pilaris a benign condition that presents as small, rough, red, or tan bumps primarily around hair follicles on the upper arms, legs, buttocks, and sometimes cheeks. It is usually a cosmetic issue and not harmful. You can try moisturizing lotions to soothe and make it look better.