My son keeps getting red circle marks on each side of his mouth. What is that?

Angular cheilitis. This is result of saliva collecting in corner of mouth and allowing yeast to form. A topical anti yeast cream such as micatin (miconazole) coukd help. Avoid lollipops, sucking candy. He may lick his lips as a contributory cause- for that try brutes bees products. Mycolog ointment is the bes prescription for angular cheilitis.
Contact dermatitis. Some people are very sensitive to the extra whitening agents in many toothpastes and gels. Try plain colgate (the old fashioned one) for 1 week and see if the chelitis resolves. Other etiologies: lip licker's dermatitis, secondary bacerial or fungal infection.