What's going on?

The speculation is that He Tian left at the end of high school (looking at the recent chapter where He Tian found his school name tag that Little Mo kept) and the reason for him leaving was to go abroad to make a living for himself so he could return to Little Mo and provide for him (which is what we are seeing now is He tian has returned). There are no chapters that officially show him leaving. If the author decides to go into the past again, we might at some point see his departure, but potentially not.