Hate to say it but have you called the roads department or Animal Control, whoever picks up deceased animals off the road.
I would go into the usual, you can leave tuna/salmon out, check early morning and early evening. Leave something out that has your smell on it, or even his litter box. But these are tips for cats that are indoor cats/scared and won't venture far from home.
You let your cat outside. So he is not afraid and would come home when he wanted.
Good luck in finding. Keep us posted.