Learn Russian: Он устаёт поздно вечером.

The dictionary form of this verb is уставать. To form the past tense for a masculine subject like он, drop the -ть and add -л, making it Он уставал (He was getting tired). However, to say he got tired as a one-time completed action, Russians use a slightly shorter verb, устать, which becomes Он устал.

Why does the word for evening (вечер) end in -ом here?

To say when something happens during a general part of the day, Russian uses a special ending (the Instrumental case) instead of prepositions like 'in' or 'at'. By adding -ом to вечер, it changes the meaning from 'evening' to 'in the evening'. You do the exact same thing with morning, making it утром.

Could I use the Russian word for 'in' and say в вечер?

No, because the -ом ending on вечером already does that job. Adding в (in) would be repetitive and incorrect. Just use the time adverb поздно (late) right next to вечером.

I notice the letter ё in устаёт. Does that affect the pronunciation?

Yes! In Russian, the letter ё always carries the stress. Whenever you see it in a word, you don't have to guess where the emphasis goes, which makes pronouncing устаёт much easier.