You can tell him something like "Mum's boobs are very tired at the moment, you can have some at bedtime" - you don't have to make a hard-and-fast rule.
And anyway, most of your rules are going to change as he gets older (like going from "don't touch the microwave" to "do you want to help me cook" to "make your own lunch").
He needs to learn that you don't always get what you want - and that's a hard thing to learn! Distracting and keeping him busy helps - I never got asked for boobies when we were at the park, but if we were home and bored then it might be frequent.