Most Helpful Guy
It can be a bad sign depending on the context of the situation. I remember when I was an arrogant sod at 16 and I had this girl wrapped around my finger, I dumped her and when I realised she had moved on I got in contact with her again and I'd smile when I saw her, it was mostly because deep down I liked the (stalking-like) attention she was giving me as shameful as that sounds. I'm 19 now and realise I was a horrible person and I told her upfront we shouldn't see each other anymore over a year ago. I'm not sure how you want to apply my opinion to your question but take note, if you want him back, my advice DO NOT MAKE IT EASY FOR HIM, especially if you or are in the process of getting with another guy or you assume he thinks this. If you get along, fine, but make him wait for anything "romancy". Good luck :)
Most Helpful Girl
I don't think you'll ever know the true reason of whats going on with his mood. He seems to be getting comfortable with you again, so maybe us this opportunity to ask him why he seems so upset and show your concern for him.