iPhone 4

I just got an iPhone 5s. I kept my iPhone 4 so my son could use it as an iPod touch for now. I will eventually have him added to my phone plan. Is there a way he can use the deactivated iPhone 4 to send and receive iMessages until I do have him added to my plan? Please help! I have reset the phone 10 times to try and do it and it won't work for me.

Thank you.