SPOILERS
Yes, you're right! Idk if it was mentioned already but Dohyun is an orphan. And I believe Yeon's mother is a benefactor. So when she asked Dohyun to take care of her son and also report back to her, he did just that, but did not expect to catch feelings. That's why he went on his military service afterwards.
Yeon's mother contacted him afterwards and he did met up with her, but eventually confronted the mother (cos the mother was only worried about Yeon but can't even talk to her damn child) and stopped meeting her after Yeon found out about it. Yeon was so hurt when he found out

He whispered about looking at his phone…theory!
One of two things is either in his phone
1. Dating him was a bet or cheating
2. Ml is obsessed with mc
I could think a few other things that could be on the phone but those are my big two