'he naturally tends towards a slightly scruffy /eccentric /academic look,which I'm not sure is quite right for the Us market IYSWIM'
You are absolutely right here. However, it might be mistaken for 'eccentric British genius' and he might actually impress some people. But all in all, it is a mistake to look more down at heel/dothery than you actually are with lawyers.
You should always put your best foot forward, appearance wise, with lawyers. They size you up when they first meet you and are pretty good at estimating the total value of your clothes and from that deducing how good you are at whatever it is that you do for a living. From that they move on to deciding whether to respect your opinion or not/hire you, etc.
Lawyers wear dark suits. The prosperous ones wear very nicely cut suits. They all tend to wear wing tip shoes, never brogues (the men anyway) with no scuffs and not down at heel. Very lawyery shoes here. Black socks goes without saying, and not chunky black socks either. Fine knit, but not men's pop socks.
He will need a very good quality black belt with preferably a subdued finish and not huge buckle to wear with his suit trousers.
He will also need a nice wallet, in leather, and not looking as if it has been his since he was 18.
A decent pen wouldn't go amiss.
The waistcoats from the WSJ might be a little ott but otherwise that is a good guide.
Ties need to be expensive looking and not necessarily conservative. Splashy is ok (depending on the sort of law practice -- commercial litigators and entertainment lawyers can be quite out there) but they have to be expensive looking. Look at the details of where the ties are from in the WSJ page, and check out the prices. A good British tie from some decent menswear place would be very acceptable. Not M&S. The tie really needs to be silk.
No button down shirts. They are not in any more and haven't been for several years. Good quality fine cotton is acceptable and in a light colour or white. Nothing that looks faddy like the current really wide set collars. Under the shirt he needs to wear a white undershirt that has short sleeves. (like a vest but with no hint of skin showing through the shirt even if jackets are taken off at meetings or dinner.) The undershirt should not show at the collar.
A good wool coat in a dark colour, very plainly tailored, with lapels, would suit NYC in November. NO casual jacket.
A scarf in a sombre colour would look good too. Gloves should be leather. He should have a smart briefcase, nothing blingy. Lawyers carry around massive leather document holders for the most part.
He needs to get a very sharp haircut, and it wouldn't hurt if he has razor burn or any other neck skin issue to get that seen to. Nails should be manicured before he leaves if he really wants to put the icing on the cake.
Excellent anti-perspirant/deodorant goes without saying.
NO NO NO to the sweater for dinners out. A business dinner out with lawyers would be dinner at a place where suit and tie would be de rigeur. There is really no such thing as a smart sweater for business dinner wear.
NO NO NO again to the cheap watch. There is a middle ground between Cartier and Timex and he needs to find it. Fossil doesn't look too bad. There are some fancy Citizen watches between £100 and £200.