He could well be unbearably smug this evening - be warned.
If the female is highly territorial, you may face the prospect of her coming in your own cat flap after your boy at some point so it may actually be necessary to keep him in all night in future (depending on events) while locking the cat flap. You can also get cat flaps which only react to programmed microchip codes if he's got a chip. They're a bit expensive but ....if needs must.
I wouldnt put a cat of mine out at night, it's the worst time for RTA's.
A couple of litter trays is what he needs really, mine wees where he shouldnt if he is upset. He and the other cat should establish a hierarchy eventually when they have decided who is boss my monies on the female winning that battle
Myself, I'd get him a litter tray directly and keep him in for a few nights rather than putting him out. He must be stressed and unhappy to have done something like that out of character and the whole going out situation (other cat and at least one catflap he's not keen on) sounds as if it's just too much for him.
He should be able to learn how to use a tray fast: just put him in it and do a quick scrabble in the litter, holding his paws, to give him the idea. (He probably used one as a kitten so it shouldn't come as a shock.)
Then - lots of love, affection and taste treats. The whole TLC routine.
I'll leave it to others to recommend on positioning any Feliway.
This doesn't mean he'll never go out again - just giving him the option to stay inside with you and use the facilities there might give him enough confidence to start going out occasionally to see what it's like; knowing that he doesn't have to if he doesn't want to. And build from there.
We have a 2yr old neutered male cat, fully house trained and generally lovely and snuggly. We've recently moved into an upstairs flat in an old house - so he has to go out of the catflap in our door and then cross the communal hallway to get downstairs to the catflap that goes outside.
But, there's a bossy female cat in one of the other flats and they are fighting over the hall [sigh] She does lots of sitting outside our door, giving poor old Tiger the evil eye if he tries to go past her and hissing at him.
Last night, I heard our cat mewing, so got up and put him outside our front door (there's a catflap in it but he isn't that keen on them yet). Went back to bed, and when I woke up he was asleep by my door. And there was a turd on my bedside rug, and my bed itself had been pissed on while I was in it
So... I guess the other cat didn't let him go past past night. He is being put out at night for the next few nights and the catflap locked but that's not a long term solution. Am thinking of Feliway but where to plug it in? Would the hallway be the best place, as that's where they are battling and sitting staring each other out? Would a litter tray inside be a good idea or would it confuse him?