If he really "cared" about you he would stop tickling you. Obviously you can't make him care. If he did he would honor your request to stop. My question to you is why continue to stay with him? There are other more caring individuals out there.

I am very ticklish like that too. My older brother used to pin me down like that when I was little and tickle me until I cried. It is abuse and you should tell him he is abusing you as much as if he hit you in the face. It's a violation. Pinning you down itself is wrong and I'm thinking probably criminal as well. You boyfriend has control issues and if you think on it a little this is probably not the only evidence of it. If I think about being pinned down like that again I get a panicky feeling. You should be able to trust this man and you can't. What else might he think it's ok to subject you to that you should just get over because he thinks it's fun. Lay down the law and if he does it again leave him. You can't marry and have children with a man that enjoys controling someone that can't fight back.