nick,

Being assertive starts with a total understanding that you are sovereign over your body. You have every right not to be hit, playfully, psychologically, physically or otherwise. Its your body, so his jabbing is invasive.

From that ground, perhaps you could say calmly:
“I don’t like that, stop. Now.”
“If you want to touch me, how about rubbing my shoulders?”
Get up and leave the room
“When you do that, I feel abused. Is that what you intend?”
“That doesn’t feel loving”

The more calm you stay, the more likely it will be to reach him as long as you are consistent. It is quite usual for a jokester/abuser to be doing what they are doing to get a reaction. If your reaction remains “that is wrong” without an emotional outburst, there is a much better chance that his desire to poke will blow itself out. Said differently, if you are consistently reflecting it back at him, he will either see it and stop, or you will become more and more assertive until you realize his cuteness isn’t enough to keep you in the line of fire.

In between, when he seems open and with you, you could try explaining your side of it. Don’t tell him what to do or what not to do, but say that when he does X, you feel Y. You don’t like feeling Y, and it is painful. If he cannot hear you, or makes excuses, perhaps you could suggest couples counseling. Communication is very important for intimacy, and if he isn’t listening on such an important issue, there are probably many areas in your relationship where you are not heard.

With warmth,
Matt