Sunburn

What kind of mom forgets to put sunscreen on their kid. Ahhh yeah that would be me. My very pale white son Elisha has a sunburn on his back that is now 2 days old. When I took off his shirt right now it had a sunburn blister that broke open. I have no idea how to treat it. I have never had one of these nor have I ever seen one. I just put neosporin and a band aid on it, any other suggestions???