I find that saying 'do that again, it turns me on' followed up with sex noises every time you're tickled (accidentally or purposefully) works a treat.
Alternatively, the next time you see him, climb on top of him, look him in the eye and say 'Remember that time when you tickled me? That was hot. do it again. Now.' Follow this with sex noises.
There's no need to say 'Hello there, I have a non-standard sexual interest which claims a international following of internet folk who mostly can't spell but nevertheless enjoy beating off to videos of girls being tied up and tickled in strange costumes, so here are some restraints and if you would kindly tie me to the bed and tickle me for three hours I'd be obliged'.
That is completely unnecessary.
I understand your reticence. I'm very cagey about the whole business, and sometimes that's wise if it makes you feel particularly vulnerable. The notion that we must trust our partners with every scrap of knowledge about us as soon as we start seeing one another is complete crap. But let's stick to the rant at hand.
Having said all that, the longer you leave it to tell him explicitly what you want, the more likely it is to be a bigger deal, in your mind and his.
In my experience, most non-believers find it sort of amusing and endearing, but also assume that everyone has an upper limit and tickling is for minor foreplay (not a technical term).
Also, men are (well, should be) aware that if you're fighting them and telling them to stop, you probably mean it, unless you've expressed an interest in such things. Nobody likes a rapist, as they say.
And here follows my most successful 'reveal' story, apart from incidents involving people I've purposefully met for tickling.
It was a dark and unpleasantly hot night in Laos, and I was shagging a nice American boy who I'd been hanging out with for a few days.
T'was our last night in our little town by the Mekong. He was buggering off to Cambodia the next day, and I was heading north to a fabled archaeological site.
It had been a few weeks since my last sexual encounter, which was such a horrendous clash of genitals that I had gone so far as to instruct my American that if he fucked me badly, I would kill him. After hearing a detailed account of his predecessor, he agreed that this was fair enough.
So there we were, and it became clear that he was from the sex therapist school of foreplay, which involves making everything sort of gentle and tickly. Nae bother. The festivities had in fact started with a tickle fight, suggested by him (subtly suggested by me), to settle a dispute over the last Beerlao.
The poor bastard was so convinced that tickling was a sign of Not Really Enjoying Stuff, that he kept apologising and attempting NOT to tickle me. I was at the end of my Bad Sex tether, and didn't think I'd see him again, so I said 'Listen, I like it when you tickle me. It turns me on. If you sit on top of me right now and tickle me ruthlessly, I'll scream and fight you and tell you to stop, but I'll still like it.'
So he did.
I put him on a truck to the nearest airport the next morning, and wandered happily into a cafe for breakfast, already consigning him to the Memories of Really Great Casual Sex part of my psyche.
We did keep in touch, sporadically. Six months later, he was in London and extended his trip for a long weekend in Edinburgh. I met him at the train station and, as soon as we were at my place, he threw me onto the bed and tickled me. 'I've been doing research', he said. Really, that's what he said.
So our weekend involved me showing him around Edinburgh and him tying me to stuff and torturing me. And me fucking him with a huge dildo, but that's for another forum.
So, to summarise, if you want to be tickled you need to make it clear, but you don't have to make a big deal out of it.