I am doing his side quest Long Road Ahead and was at the part "Continue traveling with McCready" when I came across another companion. I sent McCready to Sanctuary Hills I'm like 90% sure. He is not there now to finish the quest. I have checked every settlement I can think of and looked in the Third Rail and various other locations and he wont show up. I've tried ringing the bell over and over, sleeping, waiting, traveling across the board and back. NOTHING IS WORKING! and when you go the selected quest Long Road Ahead and use the "Show on map" button it only shows you where you are currently standing and there is no map blip for the quest. I have Googled this issue and a lot of other people are having almost the exact same issue I am. Any info you have would be greatly appreciated.

I recommend you do not dismiss McCready and take on a new companion.
And, when dismissing companions, you don't have to send them anywhere...
When that location screen appears, just 'cancel' (for me on PC it's Tab) and companions will go back to wherever they are from.
For example, Piper goes back to her newspaper and sister.

I had this issue once, but I found him at the Third Rail -- not where he was originally, however, but instead sitting on a couch in the main room downstairs. Good luck!