There is no quick solution. Try to be good to yourself, change your diet, sleep a lot, and wait. Pray for him if you can.
In the goodness of time, a week, a month, or three, you may see what was in his head in wanting to leave. There might be a sudden revelation about his actions, his immediate family, his friends or work, that will lift the veil, so that you can smile with him and understand his stresses and strains. If you are a believer, the Good Lord does not ever give us any more than we can bear.
One fine day we shall all be together again. I wish you calm and peace.