has he got an addictive personality? If so it may be a manifestation of his addictive behaviour : the excitement of a new relationship, all the flirting etc, is an escape from the drudgery of normal life. That you or I would just put up with.
Is he doing it when he is under the influence of alcohol or drugs?
Is he showing signs of guilt or sorrow after you have found this evidence a second time?
I'd take things slowly. Talk to him about why he has done this for a second time.
Say how hard you thought you had BOTH been working at your relationship after the first revelation of infidelity.
Tell him that if he wants to save and recover his relationship with you, he has to go cold turkey and stop contacting other women, end of.
He has got you to confide in, talk over problems, and to support him. He does not need this/these other women. Being in touch with them is making the situation worse.
Its a simple choice, a real relationship with you, or an escapist fantasy relationship.
He may find it hard and painful to go cold turkey, which will be hard for you too, but that is what he has got to do. Explain to him he cannot have both of you, its one or the other.
He must stop contact completely to allow himself to move on to a better life, and also this "friend" he has been txting.
Explain to him, that they are not his friends They do not wish his primary relationship well. They have friends and support networks of their own and he should allow them to use there friends not HIM.
It will be hard for him to change, but he can. Stick with it for a couple of weeks longer and then reassess. If he is making a massive effort again to change then good.