Learn French: Lors de son voyage, il a vu la mer. - During his trip, he saw the sea.

En and dans are used for measuring time, like saying an action takes three days (en) or happens three days from now (dans). Here, we aren't measuring the duration or the starting point of the trip. We just want to say 'at the time of' or 'during' an event. For that, we use lors de.

Could I say pendant son voyage instead of lors de son voyage?

Yes, absolutely! Both translate to 'during his trip.' Lors de is a little more formal and is strictly followed by a noun to point to a specific event. Pendant is more common in everyday speech and focuses slightly more on the duration of that event.

Why use the passé composé (a vu) instead of the imparfait (voyait) since a trip takes a long time?

Even though the trip took a while, the action of 'seeing the sea' is presented as a single, completed event that happened at some specific point during that trip. If you wanted to say 'he used to see the sea' or describe the ongoing background scenery, you would use the imparfait (il voyait la mer).

Does son mean 'his' because the subject is il?

Not quite! In French, possessive adjectives agree with the thing being owned, not the owner. Son is used here because voyage is a masculine singular noun. If the subject was 'she' (elle), it would still be son voyage to mean 'her trip'.