Under 21 football is a good level of skill but it's not overly competitive so I think sending youngsters out to play non league men's football is a good idea. He will have to get used to playing against big strong defenders and although the standard isn't great at least it’s competitive and the result actually matters to those playing.
You're mistaking Worthing with high level non league, this is Peacehaven, East Grinstead and Three Bridges level, it's hardly more than County League style and level, how can you compare Premier League U21's central defenders as somehow inferior physically to Worthings and if you are its some condemnation of the Premier League Under 21's level and environment and its multi million pound budget.
Thats the point, unless you are adding to the experience of the Cat 1 environment then their is no advantage to the player other than finding playing time because their are no current options within the current system.