Not to worry. Cold, chilly weather does that. Need to keep it moist with aquaphor, a and d ointment or vaseline or the like. At times if the affected areas get inflamed, a little bit of steroid may help.
Likely milkd eczema. Infants and young children often get these rosy cheeks. Not a signof health but rather of dry skin. Try a heavy moisturizer like nivea, lubriderm, cetaphil, aquaphor or even vaseline to the cheeks 2 times a day for at least 2 weeks. If very rough dry skin and no improvement, then see your pediatrician to talk about possible topical steroids for children.
Possible virus. Rosy red cheeks can mean that your son is hot, or cold, or have a case of atopic dermatitis (eczema) or he could even have a case of fifth’s disease, a viral illness which is characterized by red cheeks and a fine lace like rash on the arms and legs that comes and goes over a few days to a week. Check in with his doctor if he has any breathing difficulties or fever, or is acting sick.