Suggest Treatment For Rashes In The Waist Line

My 10 year-old son has developed a light rash around his waist. He says it hurts/itches all around, but we only see a light red line from side to side on his front. It's wider and redder on the sides. I haven't changed detergents. The clothes aren't new. Is it possible that his clothes are just a little tight? I bought new underwear for him last night, and he says they still feel tight in the waist, but he's not a chubby guy and they appear failrly loose to me.