I adopted a 3 month old Aussie. My parents refuse to allow him in the house, so he's always in the backyard. Of course we always go and take him to the park, play with him, and introduce him with other dogs in this neighborhood (he has friends!). My cousins visit him at the park, and well, he gets a walk (vigorous activity) three/four times a day. Out backyard is sectioned off as such (tiles, rocks, and grass) we have a big backyard so he always plays, and lots of chew toys and a bed.

At night, he sleeps in a bed in the garage, with my cats, who separately sleep in another bed. This is where the problem comes in: the puppy poops in the garage! His last meal is around 4/5PM and we take him out till about 9ish, playing and running. And yet he doesn't use the restroom till about 2AM and nobody is awake and we can't watch him in the garage, and AHH!

It sounds like you're doing everything right so far, with the exception of one thing ... you aren't training his bowel.

To train his bowel you will need to crate him at night for a while. Dogs don't like to soil their bed, so confining him to a crate will illiminate his desire to poop during the night. The only time a dog will soil its bed/crate is if it's totally unavoidable ... such as when they're sick.

Crate him for several weeks, then try him again without the crate and see what happens. If he doesn't poop on the floor, then you have successfully trained his bowel.
thank you, ill try that. the last time i bought him a crate, he cried and howled and i felt like i was torturing him so i took him out and returned the crate. now he happily/soundly sleeps wherever he pleases without bars.
He probably will cry and howl when crated, until he gets used to it. Get yourself a spray bottle or a water gun. Every time he starts to raise a fuss, spray him and say, "NO!". He will learn that making that noise in unacceptable.

Be sure that the crate you purchase is "just" large enough for your dog. He should be able to stand comfortably, without having to lower his head, and it should be just long enough for the entire length of his body. If it's too large, he will soil on one end and sleep on the other, and you won't be able to train his bowel.