Show them what they are missing. Talk will not do anything. I am not a PUA but I approach women with no problem. One night I danced with a few girls and my friend was just watching me. Then I saw one of my girl friend on the dance floor. I invite her to join me and she cames. I told her to bring her friend and I made my friend dance with her. At first he didn't want and I took his hand and put them on her hips.
Since then, he is my dancefloor buddy.
You just have to show them what they are missing. The hardest part is to try once. From then they will see it's easy and fun. More you tell him to approach the girl more he will thing that if he fails you will laugh at him because everyone is watching (it was my case). You have to do like it's a normal thing and not make the situation uncomfortable for him.Good luck.