Upvote:19
From May 2019, US citizens can use the ePassport gates at UK airports to clear UK immigration. It's most likely that he simply won't speak to anyone if he goes through the gates (unless he has something to declare at customs). This is in fact what happens to most people.
He might still need to speak to someone if the gates reject his passport. For instance, if the passport chip is broken or he has an adverse immigration history (i.e. previous trouble entering the UK). In this case the usual advice applies: Answer exactly what is asked using brief answers and don't volunteer information. (If in the rare event he is taken to the back for a secondary interview, that is the time for detailed explanations.)