(Post-game setting)
Sometimes Dave likes to be small. That's the long and short of it, he likes feeling tiny and protected and like nothing will hurt him. Eridan doesn't fully understand the mindset- but he understands the feelings.
It's perfect.
Eridan blows so much money to win the seahorse but it's worth it to see Dave's delighted expression. He takes it home and tells Dave he can have it soon-
And then, a couple of days later, Eridan brings the plushie out again- and Dave sits upright and blushes hard because Eridan's made a change- there, in the seahorse plushie, Eridan's added a fleshlight.
Dave quickly climbs onto it and straddles it and squirms until he gets
Dave humps like a good little boy until he spills into his new toy, clinging to it and mewling, panting and trembling. Eridan lets him shiver until he can let go-
So Dave lets himself be shushed and now he has something to work towards for making his daddy happy- and every time he fits another one in Eridan is /so/ proud of him and praises him so much.
Sometimes Eridan combines the plushes together, but no matter what he uses, as long as the starfish is on it /stays/ on- since it's got tiny little locks on the straps,
His daddy can keep him nice and full for as long as he wants and Eridan keeps the key on a chain around his neck. Dave could grab it and get himself out- but he doesn't because he's a good boy for his daddy.