Maybe you could pretend to be tied up the first few times? Like, hang on to the headboard or something, try to pretend you can't move. Finding out how difficult this is for you will give you a good indication of what to expect when you're tied up. If you're able to keep from pulling away going "oh my god I can't handle this", and can instead just lay there and laugh your ass off, then being tied and tickled for real should be about the same. If it is very difficult for you NOT to pull away, then you know you don't (yet) have a very high tolerance for being tickled. If that's the case you could either keep doing the pretend tying up and see if you can gradually work up more of a tolerance for it, or you could move on to the actual tying up, your boyfriend now being aware of what you can tolerate.