Young single men tend to draw the most scrutiny, because they're among the most likely to be entering with intent to overstay a visa (or visa waiver) and work illegally. Assuming he's a U.S. citizen, he probably won't have much of an issue, but it's probably better to be safe, buy a refundable return, and then refund it after he's entered the UK. If he's entering on the UK equivalent of the visa waiver, he needs to leave within 90 days or he will have problems in the future.