He just needs to learn that pooping is for the litter box, and if he has to go you want him to try for the box at least.
I would put a second litter pan near where you're sitting so he can have the option of using one close by. then take the poos he's made on your lap and put them in that pan as soon as they appear. If he just did it, scoop him into the litter pan too, and praise him. "Good boy!" Put a little hay in that pan for him to munch on so he will go there some more.
You want him to get used to pooping in a pan, and one of my bunnies needed this outer pan for when he was playing in another room and didn't want to make the trek back to his habitat. My current bunny, Sammykins, is very neat and she always ran back to her pan no matter where she was. It's most important for training to pee. Poos will become a regular decoration in your home over the years, I'm afraid.