The Dragon Cohort feat is in the Draconomicon (p 104) and gives you the Leadership feat from the DMG, except without the tiny minions and you can treat the dragon as if it was three levels lower.
By my calculations, Kapek is level 18 and gets a +2 bonus for Great Renown as well as a +1 for Special Power. Obviously you okayed those two modifiers, but there may be others you think apply (DMG p106). Assuming this total holds, 21 gets him a Cohort of ECL 15 (according to the Leadership table in the DMG), so for a Dragon it will be actually ECL 18.
This gives the following options from the Draconomicon:
Young Silver dragon (CR 7)
Young Bronze dragon (CR 7)
Juvenile Copper dragon (CR 9)
Curiously, my preference would be for the Bronze, since that can move and breathe freely underwater.
Thoughts?