I met Valentine and then said he could be my companion. After the mission ended Dogmeat was gone. I'm really sad because I want Dogmeat back. Anyone know where I can find him??

Try Red rocket garage

You should have been able to choose where to send him.

If not, I believe his default location is Sanctuary Hills (could also be the garage you found him at). He's usually in one of the few doghouses found in back yards there.