I agree with many different pieces of advice.
Use cool water to help prevent further burning. Don't use lotions or anything that will act as a barrier for the heat to escape the skin.
Call the pediatrician and see what they recommend. It's really hard to tell what level of burn it may be. But, a lot can be told by how he's reacting. I used to nanny for a girl who put her hand on the grill while her dad was cooking (she was 2) and got a nasty burn. She went to the ER but was well enough to play the next day.
Still, have medically trained eyes/ears help you know what to do next.