Get right on his back wheel. He's a big guy, he can cut through the wind for you. You will need to put out about 60% of his effort to stay behind him, even at speeds above 20mph. If he wants to work hard, make him.
Mechanically, I would just suggest tires. The 700x32 S-R3 on the bike are pretty hefty, and built to be tough and inexpensive. A more supple, quicker-rolling set of 700x28 tires certainly wouldn't hurt.
My wife doesn't have the luxury of riding nearly as often as I do-- she's lucky to get in 5-6 rides a month, I ride 5-6 times a week. So when we ride together, I pace it appropriately. Maybe bring that up to him-- he doesn't always have to go 18mph.
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