There’s lots of hotels in Kings Cross and it’s central enough for getting around. He wants Soho for bars / pubs. Many quite authentic for Central London. Camden has gone down hill. The markets sell ironic t-shirts.
I’ve lived here on and off for 15 years.
Borough Market and the old pubs around the South end of London Bridge is far better than Soho. Camden Market is busy but a fun place to visit. Good pub by the canal. Good cheap hotel is south of Waterloo Station. Travelodge. Easy to get anywhere from there.
As others have said, Hyde Park for a nice walk, Harrods is nearish.
Use the tube for train station to hotel but if you are central a one day pass means you can jump on and off buses when you like. Most tubes take 5-10 minutes just to get down to the platform. But do them once at least for the experience. They accept contactless so you don't need a pass.
South Bank walk from the London Eye takes in a few sights plus there's usually a book fair, market going on of a Sunday.
The big sights can be done in a few hours amble. London Eye, over the bridge for Parliament, Westminster Abbey, the old mint, Elizabeth Tower that has a big bell in it called Big Ben, up through Horse Guards via Downing Street, out the back to The Mall and St James Park and then Buck House. Up through Green Park past the Ritz, afternoon tea? and then Trafalgar Square. Then onto Picadilly Circus and into Covent Garden.
A few hours walking and you've been a tourist.
There are some great back street pubs in between Westminster and Mayfair.
Boat trips go from Westminster Bridge, Waterloo etc etc down to the o2, formerly the millennium dome. Its now full of shops/bars etc etc and you can also walk up onto the roof. Pre book I think.
Soho is gentrified now but still a nice place to amble around. China Town is lovely in the evening. All lit up by lanterns.
London is a great place, lots of parks and hidden gems.
Enjoy.