I just got a new kitten and he hasn't gone to the restroom yet?

This is normal. They get stressed in a new location. Just take him to the litter ever few hours and leave him there, he will find his way out. Ours didn't go for 36 hours after we got her home.

sounds like anxiety adjustment.... 12 weeks I am not nearly as worried as younger. Even though he is fully litter box trained, the stress of adjustment may have him finding an alternative location for a short time. Watch him & if you believe he still hasn't gone by morning, then please take him to the vet

Totally normal, with my new kitten she made it almost a full 2 days without going, but she's going just fine now. keep putting him in the litter box, he's just trying to adjust.

He just needs to get used to his surroundings and relaxed.

Also it might pay to check that his litter box hasnt been used by any other cat (or dog) even if it was ages ago because he might still be able to smell it. My cat wont go near another cat's box. I have to clean it out with disinfectant EVERY time she uses it or she wont go near it if she can smell anything. Don't use ammonia based products though or they still wont use it.

Also if its a small kitten sometimes it helps to wet a cotton bud with warm water and rub their bum with it. Apparently it simulates a mother cat licking the bum of its kittens to make them go.

This may or may not be a problem but if your worried you can simulate what a momma cat would do. Take a warm wet paper towel and wipe the cats genitalia that should stimulate the cat to go. If that doesn't work you might call your vet to get more suggestions.

He's in a new place and doesn't know where the box is. Take him there every couple of hours and leave him there. He'll get the idea. Don't worry too much about it. Once he's sure he'll be safe in the new place, he'll use the box.

He'll be fine. Just a little nervous right now with the new surroundings. You will probably have a nice little "present" in the morning.

Give him some more time. Maybe a full 24 hours...he'll go by then if he's eating and drinking okay.

Place the Kitten in the Litterbox several times a day it will learn it's their instinct....

Did you look behind the couch? Just kidding.....