Two males attitude change after one got outside.

I not normally a cat person nor so I know much about their behavior but here is the situation. We have a 1.5-2 year old male bynx who is neutered and then a 8-9 month non neutered male named loki. They have gotten along the whole time we had them. The younger one from a kitten and the other from Probably a few months old as well both rescues. One night it was extremely hot in the bedroom and and I had opened the windows while half asleep... Well the screen in the one window was up and bynx got out. Didn't know at the time till my fiance called me and said she couldn't find him. She went to set food out for him incase he came back during the day and he was on the porch. Now anytime bynx and loki are near each other all they do is stare hiss and attack each other. It's gotten so bad that bynx has been hiding up stairs while loki rules downstairs. Any help would be greatly helped! PLEASE HELP ME OUT HERE. Loki has to be put in the dog kennel so so that bynx can even come eat. :/

Since your boy got outside, he now has smells differently.  He could have the scent of other animals or cats on his body and the young one is reacting to the strange smell.  Take a clean washcloth or old sock and rub the one who didn't get outside all over with it.  Be certain to get under the chin and on the pads of the feet as these are the location of scent glands.  Then take the cloth you just rubbed the young one with and rub it all over the other cat.  Be sure to get everywhere.  You may need to do this a few times.  This will help the cats to smell similar again.

If this does not help, you may have to separate the cats for a bit and try some reintroduction techniques. 

Be very mindful of getting the young cat neutered as soon as possible.  He will be coming into maturity very soon.  His behavior will start to change and he will take every opportunity to get outside.  He will be able to smell a female in heat from very far away.