Linked i phones

Hi all,

I gave my son my old I Phone (4), he has a new sim card in it, I have the 5.

However, I just found out,he receives all of my text messages and I receive his.

As you could imagine, him being 8 years old, he gets some unbelievably inappropriate sexting from girl that's meant for me.

How do I stop this from happening? He didn't know how to delete texts and asked his mother, who is now the EX, and she has seen all of my personal texts.

I need help. Please!!