Ex contacts me then cuts the conversation short?

Anonymous
Since breaking up over 4 months ago I have been in strict NC with my ex. He didn't want to commit to the relationship and he hurt me emotionally by texting another girl a few days before I found out. I ended the 'relationship' and I did not speak to him for 3 months.

He then contacted me out of the blue a month or so ago asking how I was etc. We exchanged light conversation and even though he tried to prolonge the conversation I soon stopped replying (it was going nowhere and became difficult to reply to). I then heard from him again and again for a few days after that, continued to exchange (I was quite blunt with my replies, short and sweet) and then ended the conversation. He tried to remind me of things he remembered about me and he tried to be jokey, he also tried to add kisses to his texts which I didn't send back. The conversation stopped again and I heard nothing for weeks.

A few days ago he contacts me again and asks me how my vacation was (I had told him I was going away when we were together/he reminded me about it when we were texting). I replied and simply said it was lovely and thanked him. He then replied "good" and added a smiley face and a kiss this time. I figured he wanted to say something else so I waited... Nothing. I then felt guilt that I should have relied but how do I reply to that? I've not heard anything in days.

Why is he checking in on me and then cutting the conversation short? My friend thinks he's trying to test the waters and has realised the grass isn't necessarily greener. But if that's the case why would he leave it weeks between contacting me? He hurt me bad, is he trying to earn my forgiveness?
Ex contacts me then cuts the conversation short?
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