Its just his age...ie: growing-pains. Whenever a child is about to hit another new age-juncture or is in a new age bracket, it tweaks them. Its a phase. One of many.
If looked at in a biological/developmental way... then for me, punishments/time outs will not work. Because they can't help it that they are having growing-pains or changing developmentally or even teething. And then we have to choose whether to battle it out... or not. Something like sleeping or napping or bathroom or eating phases, to me... are not something to battle about.
ALSO at this age, they begin to have night-time 'fears' and also separation -anxiety (which still can occur although it is expressed differently per their age)... and we can't just turn-off their imaginations.
Sure stick to your usual routine. But how about, in a basket beside his bed... put SEVERAL stickers/cars for HIM to choose from, for bed. And even if he changes his mind, or wants more than the obligatory 2 things... then fine. Its no big deal how "many" he has... the point being that he feels some sense of security and regularity with it, and that "he" can choose... too. I don't think the issue here is "how many" stickers or cars he is allowed to have.... nor that it 'has to be' only 2 things.
For us, my son has a basket next to his bed with a few favorite toys & a flashlight in bed with him. AND, he is allowed to choose which one he wants and if he wants to choose another one, HE can. We allow it and he has been responsible about it for a little guys his age... and because we allow it, he has been more independent and happy to go to bed. And once he is done "playing" then he is fine and he goes to bed. No battles about it. It has taught him self-reliance too.
Also though, kids just get lonely in bed or their room by themselves in the dark. When I was that age, I did the same thing. It was just a phase.
No need to lock his door or put up gates. A child will learn to navigate themselves through this with a little help. They are still very young.... and it takes time and allowances & our help for their developmental tweaks along the way. NOTHING is engraved in stone. It won't last forever.
All the best,
Susan