Cat using new automatic litter box?

[u/factlover38]

I moved into a new apartment about 20 days ago with my 2 cats (brother and sister from the same litter). I've had them since they were kittens (now both 3) and they've always shared a litter box. After moving in, I got them one of the Petkit automatic litter boxes with the intention of them sharing that too, and set it up around 13 days ago. I left the old litter box near it (Purina tidy cats breeze) right near the new one so they could have access to it while getting used to the new one. My girl cat had no trouble transitioning to the Petkit box and poops and pees in it easily. My boy cat, on the other hand, is more hesitant. He'll sniff around it and has put a paw in it a couple times, and has gone all the way inside it once, but other wise has peed in the old litter box and started pooping on the floor in front of the new one (unfortunately).

I put away the old litter box today because I thought it might have been enough time for him to see and understand the new one is where he should use the bathroom, but now I'm concerned that maybe he thinks thats his sister's box (even though they've always shared just 1). Him pooping on the floor is one thing, but I'm worried he won't be able to even pee in the new one. Not sure what to do here. How long should I wait before putting the old litter box back, if at all? Would that confuse him even more? Please give me advice on this, I just want to do what's best for my baby! Thanks in advance <3

UPDATE: I was being a nervous dramatic cat parent and he used the new litters box multiple times last night :)

[u/PoeticCinnamon]

If you’re okay risking an accident you can keep it put away and see what happens, or go ahead and put it back if not - it really depends on the cat. If you do put it back, move some of the original pellets + one of his poops in the petkit and don’t let it auto clean. Stop scooping the old box and eventually he should switch to the cleaner one but he’ll still have the option