Hi, welcome to the forum!
If he also added his email to the ipad, next to yours, then it depends what de default mailaddress is set to for sending mail.
You can find that at the settings/mail. At tye bottom you'll see something like default account. Choose your own and you should be ok.
Only if he sends an email from your ipad he should change the sender in the email itself before sending it, else it will go from your address.