The issue that you face is not an uncommon one.
In our current culture males are strongly socialized to 'never harm women'. It's a good cultural instinct, and in almost all cases its a rule that works as intended.
The exception is a case like yours. You want to have something done to you that from the outside perspective of a vanilla individual as 'harm'. He'll be making you move/say/etc things out of your control, and your general reactions may well sound like honest distress even if you are enjoying it to your core.
As a result a fair number of guys balk at doing the stuff.
So what is a person to do?
Talk about it. And not a little but a lot. You need to clearly say to your partner that yes it looks like I'm in distress, and I may call you horrible things when you tickle the hell out of me, but I WANT this. I WANT you, someone I care about to do it to me. It gets me off.
It's also why you talk about safe words. You give him a real word that you can say that stops everything cold of you say it. This lets him know you can stop him if it's not right or too much. He doesn't probably understand the masochistic feelings you have about tickling, and thus has a hard time knowing WHY you want it. But knowing you have a way to stop him will go a long way toward making scenes possible.
Can these talks be embarrassing? Yes, if you are just coming out in general about your likes. It feels weird to tell someone such personal things, but then this person is someone you re intimate with already, and who better?
And you also need to be ready for him to say "I just can't do that. It makes me feel badly even knowing you like it." and be ready to accept that this is a possibility. Some folks simple can't be sadistic in such ways.
Good luck.
Myriads
