We have a rescue cat who has been inside for 9 weeks now. He is 3. When we got him we were told not to get another cat as he had been in some really bad fights and had terrible scarring on his chest. His owners, rather than neutering him, surrendered him saying the fighting was too upsetting 
So he is now neutered and chipped. He seems contented for the most part if a little bitey at times. But today I think he is just bored. He has been attacking everything in sight, not maliciously but more playfully. He enjoys looking outside but doesn't spend loads of time doing this. We both work but we play with him and he has scratching posts, tunnels, balls, wind up mice, bubbles etc. But I don't know if this is enough anymore.
Also our neighbours behind have chickens and my friend pointed out that chickens attract rats and if he gets hold of a poisoned rat then he would also be poisoned.
I feel he would love to go out but I am worried about him fighting like he did last time. I don't know whether he started it or if there was a big bully of a cat where he used to live, but I want him to be safe. And I don't want him to be poisoned.
What would you suggest? DP wants to build a catio but has now decided it should be a shed with a meshed front. But this to me isn't a catio but just another room albeit outside.
So what do we do?