How Can I change my IPad bought by my son,to use my email and phone number instead of his?

You can’t change the number and email address. This has to be done by a phone service like AT&T, T-Mobile, etc., or a company that is contracting with them that sells their service, like WalMart, Target, etc. They would assign a mobile phone number and use whatever email address you may already have from your Internet Service Provider , e.g., Cox, Comcast, etc.
If you have a device using your son’s email address and a phone number, different from what you want, you may have to cancel the contract (assuming you have one) with them in order to get a new number assigned by a different company. Cancelling the contract may still hold you responsible to pay the fees for the entire contract (e.g., 2-years) you currently have, or you may have a penalty to pay. Check with you current phone service provider, e.g., AT&T. Make sure to ask about penalties and fees if you cancel your current contract. They may be willing to roll over your current contract to a new one, and not charge you penalties. Ask !