Why is he doing it? what can I do to make him stop touching me?

There is one colleague at work, we have a good relationship, we are more or less kind to each other, but I have no other interest in him. He was still a new employee when he started touching, touching is mainly manifested in a shoulder touch during a conversation, whether it is talking about work or for example during lunch, in addition, when he gave me a seat in a row, he gave me a seat by putting his hand on my waist.
Recently, when we were looking at something on the computer at my desk and I was sitting at the edge of the desk in order to avoid touching him, he was pressing his arm to mine. I am 25, he is 29. I am not a touchy person and not open others to touch me.
1. Is there any possibility that all of his actions to be just friendly?
2. what might be his intentions?
3. How to make him stop this without ruining somehow friendly relationship?
Why is he doing it? what can I do to make him stop touching me?
Post Opinion