Keeping a cat indoors while a door is open?

i have no problem to keep him indoors with the door closed, and leave him on a leashe in our backyard to explore while checking on him for awhile. but as its summer and hotter during the day my mom leave the door open and the cat escapes. sometimes i wontr be there to keep him back on the leash, or its just not practical at the moment. so will he EVER avoid running out the door as soon as it opens? male, non nuetered, 3 months old. usually he will go to a corner outside our yard into a bush and hunt insects and flys and such for entertainment but he could get bitten by anything then! anyways please help. sometimes hes locked in my room, but im afraid he will poop in here. :( thanks a lot!