Let's sweeten the deal by offering to grow him four or five of these guys;
Advanced Gigantean Bladeleaf Plantblood Manta Rays (CR 6). We just sold 32 of them to Vialesk. They're Colossal-sized non-sentient combatants, beefy, and extremely fast swimmers. He could use them to transport several of his Aquatic Trolls much, much faster than they could swim. Faster than he could swim, too, for that matter. He could use one as a mount if he assumed a smaller form. Five of them would only cost us a total of 6,000 IM and they're a very impressive gift, IMO.