Apologies in advance for the length -- this is a bit of a complicated situation, but I'm at my wits' end and would really appreciate any ideas or experience you may have.
I've had my neutered male ginger cat for 7 years and he's always been an outdoor cat and a wanderer. Our house backs onto a very overgrown park, which he adores. He has a hole in the wall between our small, paved back yard and the park, and a cat flap in the back door. His pattern until recently has been to spend a lot of time indoors in the colder months and most of his time outdoors from around May - October, popping in when it's raining or to eat.
We've had an ongoing problem with one or two neighbouring toms (not sure who owns them, if anyone) coming into our back yard. One of them, who we nicknamed The Bastard, learned that he could open our supposedly locked cat flap (one of those magnet ones) by head butting it. He would come in every night and eat any cat food we left out. We didn't want to not leave food as our cat's visits were quite unpredictable. However, we recently stopped leaving the food and fed our cat when we saw him. We also just got a Sureflap microchip cat flap which I'm hoping will be Bastard-proof. The thing is, the Bastard actually seems to have gone, but I think he must have scared our cat so much and/or marked our yard as his territory, so now for the past couple of weeks our cat has completely stopped coming home. If I go and call him in the park, I usually find him and he lets me bring him home and is obviously hungry, so he has not moved in somewhere else. However we are going away next week and I can't ask the cat sitter to search the (large) park every day -- also the cat probably wouldn't come for him as he's a stranger.
I've been taking the cat into the yard, feeding him there and trying to show him it's safe, but he hisses, seems very nervous and escapes as fast as he can. I really feel that he has "forgotten" how to come home, or given up trying out of fear. My question is: is there anything I can do to make him feel like it's his territory again?