Until a few minutes ago, I assumed he was a US citizen. But now I realize that he has to prove a 5 year period of continuous US residence between the ages of 14 and 28, in order to still be a citizen of the US. We don't know enough to decide the case. What age was he when he returned from his UK trip? Did he make other trips abroad? There is anecdotal evidnece that he visited Canada during college. Each trip abroad strats the "5 years of continuous presence" clock anew.
You can have continuous residence and still vacation outside the country.
When you travel, you don’t give up your residency, do you?