If he's been sitting on the train for 2.5 hours I wouldn't do a bus tour, fun as they are. My itinerary would be
10am - into KX
Find tube, take Victoria line (cooler than most others!) to Green Park.
Walk through Green Park to see Buckingham Palace.
From Buckingham Palace, walk through St James Park admiring the pelicans to see Big Ben and the Houses of Parliament.
Walk over Westminster Bridge, and instead of going straight onto the South Bank, turn left into Belvedere Road, where there are lunch options from the cheaper end which is Ned's Noodles to the naicer pasta places and so on. There's also a Gail's bakery if you just want a coffee and a sausage roll. Belvedere Road is much quieter / less queueing than the South Bank options.
From there it is a stone's throw to the London Eye, which is worth a ride if finances and time allow. If not, keep walking down the South Bank to have a go on the carousel, watch the street performers, browse the book stalls, maybe get an ice cream. Depending on whether you did the Eye after lunch or not, you then have a short or long option back to KX.
Short option Walk along the South Bank to Waterloo Bridge, then up and over Waterloo Bridge, and spend the rest of the afternoon in Covent Garden, where there are more street performers, some great shops / market, and the London Transport Museum (pricey after the Eye though). Take the Piccadilly line back to Kings Cross when you've had enough.
Long option Walk further along the South Bank to the Millennium Bridge, where you can see the Tate Modern and Shakespeare's Globe. Walk over the Millennium Bridge for a superb view of St Paul's. From St Paul's, walk up to the Museum of London and go in if you have time - it's free and very child-friendly - which is right by Barbican station, from which you can get the Circle, Hammersmith & City, or Metropolitan line straight back to Kings Cross.