*I want him to choose to join in...
.... Or not*
Well he will choose one or the other. He will join in, or he will not.
How old is he? Is he's sitting on your lap at a school party I'd guess pretty young. Just leave him be. Don't force him into situations he's not comfortable in yet. He'll eventually find his way. Most kids do. He might never join in for Agadoo, or join a Congo line but it is not actually necessary for survival.
He'll be fine.