Hi,
aimless traveller - I'd strongly recommend that you have your better half ONLY drive on the open highways and NOT around NYC or any city/built up areas. His experience of driving is extremely limited on a provisional to say the least, and that's here in the UK. Throw in left hand drive and driving on opposite sides of the roads and opposite ways of turning at junctions, you are only likely to increase substantially the chances of an accident in the USA.
I've often driving around the NYC area, out in Long Island kinda way, it's quite smooth and relatively easy going, but get in towards some of the freeway junctions and turns and exits it can and often does get quite stressful!
Soon as you're on a relatively light traffic, straight'ish highway swap then
- just remember to have him keep to the speed limits - I've had some great 'fun' talking to highway patrol officers plenty of times !!
Cheers, DtM! West London & Slough UK!