Can't access Apple ID as no access to old phone

My son had an Iphone 4s. I had an SE. I was given a 6s, and my son had broken his 4s, so I gave him my SE.

During the switch over, he managed to lose his sim card. As we were planning on switching him to a cheaper sim-only deal, we didn't get him another one (we would have had to pay), and just got him the new one instead.

After resetting the phone, he now can't sign in with his Appleid as he no longer has the phone (or the number) that the verification code is being sent to.

Is there a way to get around this?

At the moment the phone has been set up with my Appleid, which is obviously not ideal!