I'd assume his NFL fitness training focused on explosiveness and recovery.
The recovery aspect in NFL is the gap between plays, the subbing in and out & the change over from offence to defence. Actual running etc is only in very short 30 sec to a minute bursts, and no tackling etc. I am not sure how that will translate into 7 minutes of almost constant movement. I mean the 7s specific players are ridiculously fit specimens.
Additionally given he's not played any NFL for over a year (I think) and no rugby for 4 years he is going to be very rusty!
Good luck to him though, hope it works out for him.