Kids have different reactions to hearing for the first time. He didn't know what he was hearing, he just was feeling something he hadn't before and that made him laugh. Some kids cry, some smile, some do nothing at all.
Here is a video of a kiddo I know getting activated. He laughs too.
Meet Cal...: Cal's Activation
But honestly, this is the single most moving activation I have ever seen. It is a girl adopted from China who uses ASL, I think she is about 5, she is soooooo happy. I cried and cried.
Having trouble with the link.....hold for more info...