Problem with two iphones on the same Apple ID

I have an Apple account and an iphone 5s and my bought my 11 year old son a iphone 5c for his birthday. His phone has its own separate phone number (obviously) yet functions under my Apple ID (for security and billing reasons really) . When he rings me using his phone, mine goes straight to voicemail and registers the call as coming from MY phone number, not his (if it registers it at all). He also can't text me (or reply to my texts - well, he can write a reply, but they never reach me) although I can text him (not imessage, just text message and I can ring him. Why is this? It's a right pain as I bought it so he could communicate with me when he's at his mum's, which is proving impossible. We also end up with each other's photos coming through on each other's phones, which i guess is down to sharing the Apple ID? Is there anything obvious I'm missing? Do I need to set him up with an Apple ID (I'm worried that 11 is too young for this anyway.)