depends on the area where it turns red. if it is on the penis then i think you should bring him to the doc coz he might have urinary tract infection which can be serious. If it is the skin around the area and closer to his thigh, then maybe its just nappy rash and you can try to spread some zinc cream or nappy cream every time you change his diaper. hwvr, after you put on the cream, if the condition is the same fr 2 or 3 more days or got worsen, you must bring him to the doctor's.
my son had the same problem a few weeks back. the foreskin was very red and I took him to see doctor. I then found out it was because I did not pull back his foreskin when washing his penis and so he had an infection. The doctor gave some cream to put on and reminded me to always pull back the foreskin to wash when bathing and changing nappy. I did what the doctor said and no problem since.