I think he's just doing it for fun (becos you said esp. when there are many people) + he's not very hungry. If I were you, once he 'Lun', I will stop feeding until the next meal. I know it is easier for me to say, but maybe very difficult to carry out if it is my b. However, it is worth trying.
If I were you, I will skip the milk as well by taking him out to play then return home around the usual meal time & give him his meal pretending everything is normal