The simple answer is there isn't a straight answer.
It depends what contract he has on his UK phone and if he hasn't told his provider (e.g. Vodaphone) that he has taken his phone abroad, it won't work.
It's up to him to tell you what charges may be made by calling you and he should get some idea from the US provider of the phone service, just as your provider should tell you what it costs to phone abroad. Just Googling for help will waste a lot of time. Check directly with your phone service provider. Hope that helps.
The good news is that the 'roaming' charges are nowhere near as expensive as they were a year or two ago.