My baby's rash is bothering him and he keeps scratching it. What can I use to reduce his itchiness and clear the rash?

Aveeno/benadryl. I would consider an aveeno oatmeal bath, which can be very soothing to the skin. If your child is an appropriate age (ask your doctor), children's Benadryl (diphenhydramine) can help with the itching. Benadryl (diphenhydramine) gel or hydrocortizone ointment over-the-counter can oftentimes help an itchy rash. If your child appears ill or has high fever, it's wise to consult your doctor.
Probably eczema. The most likely cause of your child's itchy rash is eczema, an excessive dryness of the skin. It is also called atopic dermatitis and is occasionally associated with food allergy. The mainstay of treatment is moisturization of the skin. If the skin is severely inflammed a low dose cortisone cream may be helpful short term. In older infants an oral antihistamine may decrease the itch.