Our male kitten did that too. We just shut the bathroom door. He stopped. He also pooped on the floor next to the litter box. We added another box. He stopped. We have 3 cats and 3 boxes all in a row. No more poop on the floor.
That's not unusual at all, cats think a bathtub is a giant litter box! My older cat does it once in a while and I'm thankful it is there and not on the carpet! Your kitten is entirely too young to know for sure WHERE he is supposed to go, hopefully you can teach him that the litter box is the appropriate place. Don't clean the litter box TOO good either, he needs to smell where to go. You might want to place him in there several times a day, like small children sometimes they can't hold it long enough to make it to the bathroom. Good luck!
My adult cats does that occasionally, but he prefers to poop in the tub, rather than pee. He gets fed in the bathroom since he's a slow eater & the others will steal his food if they're together. If we leave him in there too long...surprise! If you can stop it young, you're ahead of me!
Hopefully it is a little age related phase. I would add another litter box and take him to one several times a day like Di and Bob suggested. Kittens are known for potty accidents; so if you can spread them out around your home a bit it might help him get to one if he suddenly finds he's got to go.