My son has developed a dry rash all over his body it looks like thousands of goose bumps. he's fine in himself. it's like clusters in places. he's 3.

Rash: Sound like dry skin bordering on eczema--it can often have a "goose bump" appearance. Heavy moisturizer, like cetaphil cream (not lotion) can be very helpful to get rid of it and prevent its return. Use morning and night for a few weeks.

Difficult to tell.: It's very difficult to identify rashes from a description on the internet. Have his doctor take a look so you can find out if is anything to worry about.