It's easy to tell if he's had enough. Soak his feet in warm, soapy water, then slide your pinkie or index finger into the crease between his big and second toes and, twisting your hand from side to side in a "so-so" gesture, run your finger all the way into the V and back out. Gently but confidently tap the head of his penis against your tongue while making smouldering eye contact with him. He was fine, but it was a really foreign experience for me.
Now you can touch his penis, with your vagina, while naked, as much as you want. Twist the rings in opposite directions moving from middle to the top and base of his shaft at the same time.