By virtue of timezones I'll beat Sebastian to this comment, but just a hat tip to him because he's made this point in the past.
There's a distinction between an act that a Dom performs "in-scene" as a way to progress the roleplay, and a true punishment.
The first involves things that are well within the limits of the sub, and that in reality the sub enjoys.
The second, the punishment act, is chosen specifically because the sub does not like it, however it has to be something that is agreed to be within the hard limits of the sub.
I hope that makes sense
Let me give an example.
A non-pain roleplay act might involve some kind of ritual. If you roleplay using a specific scenario (like teacher and naughty schoolgirl) then such an act could involve writing out something humiliating like "I must not behave like a cheap whore in public" one hundred times.
This kind of punishment can be pushed further. For example, the Dom can pace around you as you write the lines out. He can lean over your shoulder and complain about spelling, or untidiness. He can snatch a partially completed paper away, rip it up in front of you, and make you start again.
A real punishment is often an act of corporal punishment that is more severe than the CP you would accept in a roleplay. For example, in a punishment the Dom might strike you on the back of your calves instead of your ass. Or on the back of your hands (careful not to strike too hard there).
Different couples have very different ideas of what a punishment entails. You mentioned that your Dom is still getting used to the idea of S&M, and so I would urge you to keep your fantasies dialled back a little at this stage. For example, one punishment scenario might involve the sub eating dog food (I'm choosing an extreme example here). This is pretty gross for many people, and if you suggested it to your Dom it could cause him to react very badly.
So let's say the idea of this as a punishment appeals to you. How could you adapt it and not gross him out?
Instead of dog food, use a can of beef stew, or meatloaf or something fit for human consumption. He could still make you eat it from a dog bowl on the floor, with you wearing a collar.
I need to go and make my wife a cup of tea now...will write more later
Cheers,
Stanley