Thank you for your query,
may be because of dry skin
or atopic dermatitis
Apply moisturizing lotion to his body twice or thrice daily and for bathing you can apply moisturizing soap.
To retain water after giving bath apply immediately the lotion.
Skin infections should be treated promplt with antibacterial applications.
Consult your Pediatrician
for proper diagnosis of the skin condition and to advise you the specific medication.
Hope i have answered your query.