Can I still use my old iphone after i upgrade?

So I upgraded my iphone 4 to an iphone 6 a while ago and now my son wants to use that for when he goes to track meets as he does not have a phone and needs to contact me when the meet is over. Thing is, i'm not sure what will happen if I do. They re connected to the same apple ID so when I turn on my old one I know the new one doesn't get notifications until I turn it off again but will I have to pay more for the cell phone bill? Will messages even work? What will happen

Yes

You are the son per your other queries! What are you really trying to pull?

Possibly not

Have you ever thought about asking about this at an Apple store?

Yes