When I looked at a one way ticket to the UK it was quite a bit more expensive than a return ticket. Leaving the same day on the same flight. Now this was for January that I was checking prices. The one way ticket was half as much more than the round trip ticket. Go figure.
I would say find the Cheapest way to get him to the States, and then worry about your other flights together.
Here's a thought, probably can't do this based on "sometimes" needing proof of a return ticket, but, what if you bought a return ticket for him originating in the states and didn't use the first leg, just the second, UK to US, leg?