i left him at sanctuary as soon as codsworth offered to join me once i liberated concord.

hes nowhere to be found. doesnt come when i ring the town bell.

isnt at my old home etc. etc.

several days have passed ingame.

Entering Lexington from the Southern bridge, there is a dog house behind the first home on the right. He is sitting in the dog house. That is of course, if you sent him to lexington when you picked up a different companion.

yeah i learnt that he goes to a doghouse in the place you send him.

ill search sanctuary for all the doghouses and remove them only leaving one.