My one year-old got hives! Anyone else have experience with this? Any tips? How can I clear these up quickly?

He’s got them all over his legs and arms and a little spot on his cheek. But not on his tummy or back and not on his little bottom.

He just came off of 10 days of amoxicillin. Not sure if a reaction can show up way later? I can’t figure out any other reason for hives. He has not eaten anything different. I had him in fleece pajamas last night, maybe he got too hot?