He wakes Coen at the start of All Good Things, and helps with Esme's medicine during Withering Away.
After Withering Away he asks Coen to follow him and train, which is the quest Like Father, Like Son.
Ending that quest by leaving costs one time segment and keeps the prologue's side quests open. Waiting with him costs five and starts the next main quest immediately — the largest published segment cost attached to a single dialogue option.
Like Father, Like Son awards the Charge Manual, which is one end of the only quest to manual to skill chain published in full.
What is not
Listed rather than guessed. These promote out only against the retail build.
Whether he can die, and when.
Whether he appears after the prologue.
Why waiting with him costs five segments rather than one — no source explains what the extra four buy.
The five-segment figure comes from one source and is the number on this site most worth checking in game: it is more than half a phase spent on one line.