Humiliation is a subjective term. Some boys love to be diapered, some love to be their Sir’s urinal, and others love to be bound in public. What is humiliating for one isn’t humiliating for another. Remembering this will help you when the sub you’re playing with wants to be humiliated. Talk with them first, before they are gagged, and see what their humiliation interests and limits are.
Pushing a boys humiliation limits can be a challenge sometimes. With a diaper, for example, they may enjoy wearing one but not wetting one. Forcing them to drink enough liquid to the point they have no choice but to wet the diaper can be humiliating for them. It is a loss of control for them, a loss forced by another. A boy may also enjoy being bound in public, but not gagged. While they are helpless to stop you, gag them in a way that is visible. They will definitely find humility in being gagged in front of others.
Regardless of intentions, you also don’t want to make humiliation into a negative experience, one that turns either the boy or yourself off. Pushing a sub past his limits should be done slowly and with their permission. Doing so otherwise can lead to problems and bad feelings. Having a good connection with the sub can usually make these things go a whole lot easier.