If my baby is getting rashes everywhere except his diaper area, could it be the detergent i'm using?

Yes: Since it doesn't involve the diaper area unless you have cloth diapers that have been washed in the same detergent it could be a contact dermatitis related to the laundry detergent. It is more likely an atopic dermatitis (eczema) which is an excessive dryness of the skin. The reason it is not in the diaper area is because the diaper area is usually moist.

Yes: Yes. You should use a detergent that is dye-free and scent-free. The detergent may be causing contact dermatitis.