WOW, I would like to suggest something to you or rather ask you something? How do you think you could have handled that situation differently? Why was it necessary to ask him WHY he was wearing the tzisis? You might not have realized it at the time, but by making that comment he might have felt that you were challenging him. It might have been better to just smile or ignore it and wait to see what develops. Obviously he might have heard something in Yeshiva that might have touched him.
When he said that he would try not to be mechalel Shabbos, you could have then responded “I respect you for trying to make that commitment and wish you true Hatzlocha with that.” He might have been angry all Shabbos for various reasons. Maybe you didn’t react the way he wanted you too, maybe he was angry with himself for telling you. At any rate, you need to chill out, take two deep breathes before responding and always respond with respect. That is the ultimate key, showing love and respect. That is the only way to reach him.
It is very telling that he even said he would try and even let you know. In his own way he was trying to reach out to you. This is a baby step but still a step. Don’t take it lightly and don’t miss the opportunity to show appreciate and admire him for it. Don’t overdo it but still pay attention and try to give him the reaction and response that he needs.