Get him a rear-view mirror. Seriously. I've been in the position of riding with someone slower than I and it's impossible for them to not feel pressured every time I looked back to make sure they're still back there. It's hard enough to avoid feeling this just with friends, let alone with a significant other. But now that I have a rear-view mirror I can just take a surreptitious peek backwards and they don't feel harried.

Besides, rear-view mirrors are just good safety equipment to have, anyway.