There's no clear age for this, each child is different. 2 or 3 year olds may understand a basic route map but to understand a detailed map, they would have to be older. At your son's age, he may not exactly understand the route but you can make him aware that he's traveling somewhere and he would look forward to it. He may even be able to point it out on the map/globe once you reach there. Google maps may confuse him at his age because there would be so many places he may not recognize. I suggest that you draw a simple map or show an old style printed map and move on to google maps when he's a bit older.
I used to tell my daughter where we are going and how long it will take from our home from the time she was 1 yr old or so. It started with trips to grandma's house where I used to draw the route saying "We are here, and we have to go this way, turn this way...Finally, this is grandma's house". I used to give the drawing to her and as our car made each turn, I'd tell her "see, this is the left turn here" and mark it for her. Initially, she used to just glance and give me this blank look but after a few days, she understood the game.
The first time we traveled in a flight to another city, I showed her the printed map and with my finger, drew an imaginary line from my house to that place and told her we are flying from 'Here to here..' . Once we reached the destination, I asked her to show me the new city and our home city and she could point out to both places on the map.
You can try something similar. But consider it to be more of a bonding activity for both of you rather than as teaching him the route/map.
