What is important for you to do first of all is to make sure that he is broken up with this current girlfriend before continuing on with him or else he will continue to see the both of you but if he did tell her he was seeing you, she may be just being friendly because she knows he is sick. Confront him about the text and the content of the text. If you do not put your foot down where this other woman is concerned he may think that he can have his cake and eat it too because you are busy with school and the fact you still live with your mother and are a single mother of two boys your time is pretty much consumed with other things in your life. When you decide to take him back you both have to let go of the past and look towards the future. Communication and telling him what you want and need from him is important during this time because you just got back together and are trying to see if you can make the relationship work so take time to get to know each other all over again and let go of the past, What he is going to have to do is make sure that he has told the other woman in no uncertain terms that he is back with you and wants to make the relationship work. If she is still contacting him he may not have told her that you are back together so make sure he does that before working on your relationship again.
That is all the more reason to call him on the text and if he tries to turn on you tell him that he really needs to tie up his lose ends (the current woman) and that a relationship is built on trust and right now you're worried about whether he truly wants to be back with you or have his cake and eat it too. If you let it go he will continue to think he can get away with seeing you both if that is truly what is happening, He very well may have told her and she just cannot let go but you have to make sure which is the case before the two of you can truly work on your relationship. In order for the relationship to have a new start you have to work out the issues from the past and the present and work on the here and now. I say you talk to him and do not allow him to turn this on you and stand your ground.
You're Welcome and if you ever need my help again just let me know.