Most Helpful Guy
That text means he is trying to have his cake and eat it too. He wants the emotional support you provide, but wants to fuck around and not feel guilty. The NRE (new relationship energy) faded, and when he took stock of what emotions grew while it was active, a mature love was not there. He does really enjoy the convenience of you. He most certainly likes you as a person. But without the super-charged brain chemicals that NRE provides, he doesn't see you as his one-stop solution for all of his relationship needs.
Saying "I don't want to be in a serious relationship" and also saying "you will always be my girl" is a power play designed to put his needs above yours, and with a lot of girls with damaged self-esteem, it would actually work. That it didn't work for you is a credit to your view of yourself and your self worth. So kudos to you. He most likely thought you would wimper along in some submissive state of despair hoping he would change his mind, which would give him some time to take a look at the field and take a few shots. If nothing panned out, he could then have you waiting (since you're HIS girl... implying possession) for him to say those big "oh, I'm sorry, I was just confused and needed some time to think. I'm better now" words that he is assuming you want to hear.
You're done. He broke up with you. You are right to want to start putting up a no-contact wall, especially for the first year or so, because your emotionally vulnerability during this transition will make you more susceptible to manipulation.
Here's the thing. You say you made it clear that if you were done, you would no longer speak. He made it clear you were done. Yet you are still speaking. You replied to him. This is a power struggle at this point. Stick to your guns. He doesn't get to lean on you for emotional support to help him get over his breaking up with YOU. That's ridiculous. You don't need to hear about his 'dreams'. You don't need him to 'check up on you'.