If he enters the mosque when the imam has stood up from bowing, what should he say?

No File Exists
Check below answers in case you are looking for other related questions:
They pray in congregation at home
Is it permissible to step over people in order to go back to one’s spot?

No File Exists
Related Answers:
A woman walked in front of our row when we were praying in jamaa’ah
Prayer in congregation is not obligatory for women, and a woman's prayer in her house, even on her own, is better
How can she save her deceased grandfather from the sin of consuming other people’s wealth?
They pray in congregation at home
Is it permissible to step over people in order to go back to one’s spot?

Recommended answers for you:
She is sad because she is unmarried and is asking for advice
More Women in Hell Than Men?
What is the right age to get children used to fasting?
Qunoot during salaah
Ruling on marrying cousins