There is no similar visa for US citizens to the Tier 5 YMS visa.
His only options would be the Tier Government Exchange Visa, for which he needs a sponsor.
https://www.gov.uk/tier-5-government-au ... d-exchange
Or the Tier 4 student visa or the Tier 2 General employer sponsored visa. He needs a sponsor (university or HO approved employer) for both.
Then there is the Investor visa and Start Up visa, but both require endorsement and substantial funds to invest in a business,