I'm starting with a short flashback back about one year.
My girlfriend, me and our 2 tomcats - Elliot (4 years old) and Fridolin (2 years old) were living in a flat share at an old farmyard, inside a small village, together with 4 other people.
There had been a catflap and both of them could get in and out as they want to.
Sadly, Fridolin got overrun by a car - needless to mention, an emotional situation. Since we had plans moving together in our own flat (no shared flat anymore), we did not get a new cat and Elliot had been alone for about 6 months.
Then, in March, we moved. Elliot adapted really quick to the new environment. We were able to let him out after 2 weeks, and he came back without any issue. We have one window open almost all the time, and he can get in and out as needed. He has already learned what to do when it's closed, walking to any other window in hope we see him.
But as the months passed by, we noticed a change in Elliot's behavior we don't like. He became slack, doing nothing but sleep and cuddling a little. Sometimes he got out, like 20-30 minutes, came back with a mouse (he always ate himself, no presents) and sleeps again.
We thought, it's the time for a new cat and around August 10th, 2015 the little female cat, Pebbles (12 weeks old at that time), came to us. She is very active, even for her young age. I've seen plenty of young cats - sure, they're active in general, but Pebbles is of her own kind.
First contact between Elliot and Pebbles had been through the window. He was sitting outside at the windowsill, while she sat inside, right at the place where his bowl stands. One easily could notice the consternation in Elliot's face. We let him in, and the little rubber ball Pebbles had seemingly annoyed him a lot.
In the first days, Elliot avoided contact with Pebbles. When she was coming too close, he hissed at her and left. He never attacked her and even if she attacked (play) him, he only pushed her back, without claws.
The next weeks, he came home less and less. Now we know, he spends the whole day sitting at the neighbors' porch, no matter what weather. He feeds himself by catching mice, which he does really well, and is almost unattractable by goodies.
Sometimes, he comes home and seemingly is hungry, like he couldn't catch enough mice that day. He eats the whole bowl at once, which usually is enough for a whole day, and leaves again. In the short span he is at home, he growls constantly, even if Pebbles isn't in sight (smell?). He also started hissing at us, if we come to close.
But this aggression doesn't point to us. When outside, he behaves like nothing happens, enjoying being stroked. Outside, he is happy as before. It seems, as he really hates the environment and want to get out as soon as possible.
This state is ongoing since about 3 weeks. He didn't lose weight. All neighbors decline to feed him. We are at the point where Pebbles has to go. We already have a trustworthy person who would take her.
But how can we know if Elliot will come home again? We are cat people. Ending up with no cat at all would be the end of the world.
Are there any strategies we could try to solve the problem?