He won't necessarily run away but he will be in serious danger and at risk from:
-Getting hit by cars, trucks, motorcycles, buses
-Getting picked up and sold to a test lab (They could care less about collars, they just take them off)
-Getting picked up and fed to fighting dogs as bait. (again they could care less about collars)
-Licking antifreeze of the driveway (its extremely toxic)
-Eating rat poison
-Licking pesticides off of someones lawn.
-Getting in brutal fights with neighborhood stray cats
-Getting torn apart by stray dogs
-Getting torn apart by coyotes, large birds of prey, wolves, foxes, or whatever wildlife happens to be in the area.
-Catching FIV or FeLV or FIP from strays and ferals. (Feline Aids, Feline Leukemia) ALL are FATAL and INCURABLE. (FeLV is transmitted nose to nose or mouth to mouth, it does NOT require blood to blood contact).
-Roaming to far and getting lost
-Getting stolen by someone else who wants to keep him.
-Climbing up a tree in an attempt to escape an attack, getting stuck and starving to death because you can't find him.
-Getting Tapeworms, Hookworms, Heartworm, Fleas, Mange
-Getting shot at, lit on fire, brutalized and tortured, thrown over a bridge etc..by cruel teenagers and other humans.
-Getting caught outside in a rain storm, hail, sleet, earth quake etc..
I can go on and on.
Cats are NOT wild animals, they are domesticated and have been for over FOUR thousand years. They don't need to be free roaming anymore then our rats, ferrets, birds, hamsters and dogs do. It is do DANGEROUS.
And if your skeptical about that list, every one of them was taken from a list posted by a veterinarian, and EVERY one of them happened to outside cats she saw on a daily basis.
Cats allowed to free roam have a life span of 2-5 years. Are you seriously willing to risk your cats life like that?
Make them close the window and if they insist on letting their cats out, they can let them out the front door so your cat can stay inside where its safe.
Also you'll have to put him in his own room for the first week as you can't just plunk him down in front of the new animals. Cats get very stressed out when they move and he needs time to adjust to his new home without dealing with fending off new animals. Put a new blanket in with him and get his scent on it, then give it to the other animals and get there scent on it and then give it back to your boy to get them familiar with eachothers scents.
If I was you I'd also insist that these two outdoor cats were tested for Worms, Heartworm, Ticks, FeLV, FIV and Fleas before exposing a healthy indoor cat to them. It is very easy for outdoor cats to get all of those when then free roaming and I would NOT risk the health of your cat like that as it is your responsibility to keep him healthy. (Unless your absolutely certain they get their yearly vaccines I'd also insist the be tested for feline distemper). You may want the dog tested for worms, fleas, ticks and heart worm as well.
I would NOT expose my cats to new animals unless I was positive they were clear of anything that could infect or harm them.
Good Luck