Hi, K.,
I'm afraid I disagree with the "he has to learn to handle it himself" camp.
I would definitely intervene. He is only five and his friends are still young, also. Children learn through imitation. Be a good role model for them, and show your son (and them) how this can be handled.
Gently stop the game and get their attention. Say, "You are all such kind children. I know you would never want to hurt someone on purpose." (They will most likely agree with you.)
Then say, "But it looks to me as if Johnny is not having fun with this game of you all picking him up. His face looks sad and upset." (They will most likely all look at your son and probably show concern. Some will probably have something to say at this point, like they didn't mean it, or Johnny never said no.)
Then you say, "Johnny, would you like the other kids to stop picking you up?" Of course your son will say yes.
Then you say, "Okay, Johnny would like you to all quit picking him up from now on. Now, can you all think of a way for Johnny to let you know when he doesn't like something? Who can give Johnny some respectful words to use when he doesn't want someone to do something?" And let them speak. And they will. They will all have ideas for Johnny, and Johnny will know that it's okay to speak up.
By saying "respectful words" it lets them know that answers such as "put me down, stupid" and "stop it, you poophead" are not acceptable. Because some children would think these answers are okay! : ) If they say anything like that, just remind them gently that those words don't sound respectful or kind, and you know Johnny wants to treat all of his friends with kindness and respect.
The key to doing this, however, is to speak calmly and gently to the children. If they think they are being yelled at, it won't work. They will just feel defensive, and some of them may even do it more.
By handling the situation this way, you are modeling to them life skills they will be able to use in any conflict that arises for them at school. You are telling them you know they are kind children, first, so they will feel good about themselves and want to be even kinder, thus treating your son better. You are instilling compassion and empathy by pointing out that your son is sad/upset. You are letting them use problem solving skills when they give suggestions on how your son can handle the situation next time. And since they are the ones who have determined how your son should handle it, they will comply with it when he does.
I hope this helps.
Blessings to you and your little one.
P.S. And I know what it's like to have a small child, my son is in the 50th percentile now for height, but he only weighs 60 lbs. He's 12. : )