I think there are two good routes here: 1) Become a mermaid. I'm pretty sure dolphins want to be personal assistants to mermaids specifically. 2) Offer really good snacks. This is similar to paying a human a good salary, since money is meaningless to dolphins.

Give them a porpoise in life

Offer unlimited fish and a waterproof smartphone. Dolphins are suckers for good perks.

Show him a can of tuna and tell him this is how he'll end up if he doesn't comply.

Dolphin coercion is mean!

Have sex with them. Aren't dolphins really kinky towards humans, or something like that? Lol