My 5 week old baby has come up in a dry bumpy rash all over his body but he is breastfeeding fine and his toilet routine is normal. How to treat it?

You should discuss. this with your infant's pediatrician. Without seeing the rash, it's impossible to advise you. Most rashes in infants are benign and will get better on their own. Atopic dermatitis / eczema can present anywhere on the body in infants. It's reassuring that your baby is feeding well with normal bowel routine, but you should have him or her seen by the pediatrician to determine what this is. :)
Atopic dermatitis. Atopic dermatitis is a skin condition characterized by dry, wrinkled, itchy and occasionally red skin. Symptoms can be reduced with hypoallergenic cleansers and moisturizers such as Cetaphil, Ceravue, etc. and fragrance/dye free detergents. Topical steroids are also helpful. This condition stems from a hypersensitivity reaction of the immune system. It is controllable, not curable.
Normal Newborn rash. This probably represents one of the many harmless newborn rashes that appears between 1-2 months related to maternal hormones or its prickly heat (which is also harmless). If it does not resolve in 1-2 days I would consult your pediatrician for confirmation and peace of mind.