hi, In my humble opionion, you must discontinue this relationship. Sorry to say that but it seems to me that your instinct is pushing you towards end but your heart is not following the direction.
Second thing is that do what he is doing in exactly the same way. He would be very much angry on that. If he is wrong, he would never allow you to have text messages of other boys and keep it secret in the way he is doing.
Third option is play dead. It means, you do not let him suspect on your intentions. Just be silent and loving to him like always. But keep an eye on his activities. When you find something serious, act at that time. Till that time, play dead.
Rest up to you, but pl do not waste your time- act wisely and immediately.
Hope it will help you.
Thanks