Dont worry he will come home! He has been outside before so is comfortable out there, he is just probably exploring all the places he saw with only his eyes before. Keep doing what you are doing, and try the flashlight idea. Put down some flour around the tuna or whatever you leave out so you can see if there are any paw prints in it when you check on it, then you know if he has been sneaking back around.
When I was a kid, my cat took off for 3 months, and a couple years ago my daughters cat excaped through the patio door ( thanks kids!) and was gone for 3 weeks, no sign of her, then one day I saw her run past the deck, I have never moved so fast in my life, I snatched her as she ran by, she did not want to be caught!
There is a realy good chance he is just fine and just is not good and ready to come in yet, it is fun out there! keep looking up, he will be back!