He doesn't seem to know he does it until I show him.he even does it some times when he's sitting on me

Ok, I'm stumped.
The only things I can think of without further details would be
1) For a longer haired dog it could be that it is simply stuck in his fur and falling off on you and your bed.
2) Not completing the job on his bathroom breaks, possibly he is getting frightened by something or some sort of discomfort, Maybe a painful anal gland.
3) If he truly does not realize what he has done, bowel incontinence.
4) Something is affecting the tone of the anal sphincter.

I would start with the simple solutions,
1)If he has long hair check there first, a good grooming will be your friend.
Unless you have proper trimmers and blades don't try this yourself with scissors EVER.
It's a good way to cut out a circular chunk of skin, and in that area that would be disastrous.
Take him to a groomer instead, let them show you how.
2)When he goes out go out with him and monitor how he acts and give him PLENTY of time to complete the job. Make sure he goes out about 45 minutes after his meal by that time they usually have to defecate.
I would also take him to have his anal glands checked, some dogs simply don't empty naturally and they can become painful and infected.
The other two are something that you will need to get your vet involved in.
If he is having medical issues they are the ones to figure that out.