Can I use iMessage over wifi if I don't have a cellular or data plan activated for my iPhone 4?

My sister gave my son her iPhone 4 when she upgraded. His device is not on a phone or data plan. Can I text him via iMessage if he is connected to wifi? I created an apple account for him and and set it up on the device, but Setting>Messages continues to say "Waiting for activation...", and he has not received the test message i sent him from my iPhone 5.