Most Helpful Guy
When a guy leaves a perfectly pleasant relationship, it means he's decided that he doesn't want to spend his life with you. He may have decided this a while ago, but couldn't bring himself to breakup. Then the holidays came along, and he was thinking "If I don't do something, I'll be in the same place next year." This isn't your fault, and he may end up regretting his decision, but he's not coming back anytime soon.
Most Helpful Girl
He could still be in grief from the loss of a child but he didn't want you to see his vulnerability or see him getting emotional. Hopefully this isn't the case but it seems as though he probably thinks he could never have kids with you or start a life after that incident, therefore, finding no future with you as a partner.
Perhaps he just wants space for awhile to grieve but he still cares about you to the moon and back. He could just be depressed that he couldn't have a child. :/