I am glad you found him.
How wide are the bars in his cage? He probably is small enough to squeeze through and until he gets bigger,he will have to be kept in another cage with smaller bars,otherwise he will just keep getting out.
Do you have a picture of the cage?
If you don't have or cannot get another cage right now,you can get mesh/chicken wire to put over the cage until he is bigger.
And just saying,you should get him a buddy. Rats do much better and thrive when they have another rat friend. He is young,so introducing him to another rat now will most likely be a breeze. But I definitely recommend getting a second male rat. Rats are very,very social.