Will using Find My iPhone use up cellular data on the sought iPhone?

To answer your question -- needs cellular data or wifi. If not when you try to track it, it will say offline.

ZütOwl wrote:

He has his iPhone which is connected to my iCloud account,

What exactly does that mean? If it means that he (or you) is sharing an Apple ID, then that is frowned upon.

To expand on what zinacef said, you do not need to "share" an iCloud account or use "Find My iPhone" to do what you want. You should use separate Apple IDs and "Find My Friends" instead.

As to question about cellular data usage: I believe (but can't say for sure that) most of the data usage will be on your end to download maps. He might incur a minimal amount of cellular data usage for you to retrieve his location.

His iPhone would use a small amount of cell data to transmit its location back to you. Since all it does is transmit a set of 2 numbers, usage is very very minimal, so its unlikely its consuming all of its data.

Beyond that, not entirely sure what you mean by " He has his iPhone which is connected to my iCloud account" .

How is it connected? Are you actually sharing an iCloud account? i.e same iCloud account signed in in Settings on both phones?

How exactly are you tracking him? Find My Friends? Find My iPhone over Family Sharing?