Suggest Treatment For Hives All Over The Body

Hello, my 2 year old son, woke up this morning and had some welts on his legs. After a while they went away. I changed his bedsheets to make sure it was not an allergic reaction or a bug in his bed. When he woke up from his afternoon nap he had them all over his body, even some on his face. He also tries to scratch them. I am still breastfeeding him. What should I do?