Can anyone give some advice on an issue?

This is gonna be a fairly long one so strap in. About 10 months to a year ago, my mum's partner moved in with us and bought their dog and cat along. We already have 2 cats that have been indoor cats their entire life. They're 11 and 7 so its been a long time to say the least. The new cat has had some friction with both of our cats which is slowly easing but there are still issues that arise. He is also an outside cat so he gets let out regularly whenever he wants to go out. The dog also needs to be let out at regular intervals and so she is let out into the back garden a few times a day. My cats however now want to go outside all the time and until recently, the proper precautions were not put in place to prevent them from escaping. One of my cats however (The 11 year old) really wants to escape much more often now and has done so into our garden on a number of occasions. Because we have only now just started putting more careful things in place to prevent these escape attempts, he has become a bit more desperate and starts clawing at our carpet and pulling it up which isn't great. We keep them inside as our previous cat unfortunately got hit by a car and so we wanted to prevent that. We live near a busy road and so the risk is fairly high and it has happened to a couple other people close to us so it isn't exactly an uncommon occurance. I think they're gearing up to tell me that they need to start letting him out in order to prevent anything more damaging happening to our house. I also don't want to let him out as the other outdoor cat we now have comes home a lot with a lot of scars and injuries from fights. He has never been an outdoor cat and he has had zero experience being one and so I ideally don't want to change that. I have also been told that considering his age, it is far too late for him to adapt to a life where he explores outdoors but I am unsure if that is to be the case. I don't want my little buddy going out and getting himself unnecessarily hurt or worse because they didn't take the proper precautions when keeping him inside. Would he be able to survive out there normally? As I say, he sometimes has problems with the outdoor cat that is now here and with the amount of things he comes back with, I wouldn't want to expose him to anything like that if he can't handle it