I have left kids on the pot as long as 30 minutes to have a little pee, if you have a hard surface out in the play area away from others and toys you can place the pot, that's what I would do, give him positive encouragement and allow him to feel a part of the group rather than isolated in the bathroom and you may be surprised, he may be surprised when it comes out! then have a big to do about it, give him a special something to take home, I have little certificates they take hom to put on the fridge, this tends to get parents more involved and excited to take part as well!