What should I pitch a client who declined our investment deals, but agreed to meet me in person for a casual dinner?

Don't pitch him anything. Find out what he thinks you should do, and ask what he's interested in doing. Maybe he'll suggest avenues for you to pursue. Maybe he'll come round and invest anyway.

But there's a good chance you'll both get more out of a casual dinner if it's genuinely a casual dinner. You get to pick his brain. Bring your eyes and ears instead of a sales pitch and see what happens.

You only need to ask him one question related to the deals you want him to invest in. " Could you ever see yourself investing in one of these deals.....ever?" If he says No then do not ask him again, enjoy your dinner and move onto the next guy. If he says maybe one day....then ask him how long could it take? 1 year, 2 years? What ever his answer is, it is.
Bring a 5x7 index card and a pencil. Write down his name and the date and ask him "can we start counting tonight?" Each time you see him make sure you bring the index card and the pencil. Never ask him to invest but make sure he see's you bring out the card and write down the date. If the card gets beat up and crumpled the better it is. After two or three "casual dinners" he will be laughing his head off and telling his friends about your little index card. My predication is that you get an investment out of him within 6 months.

Why pitch him anything? It sounds like what your offering isn't in his sweet spot, so instead of pitching, why don't you think of the meeting as a fact-finding exercise?

You can learn a lot about what someone wants by removing your agenda, asking meaningful questions and listening.