How can I bond with a previously owned guinea pig?

I adopted a four year old piggy from a friend about two months ago. He gets excited to see me when I come into the room and when I sit on the floor he likes to come over and see me. When I feed him, he'll usually let me pet him quite a bit. However, if he's running around the room and I try and pet him when I don't have food, he runs away. And when I try and pick him up to either put him in the cage or snuggle, he tries to get away. Once I am holding him he purrs and does ok. Does he not trust me completely yet? How do I get him to trust me more?