Transportation between Oxford and London

No, the tube doesn't go that far! Regular trains from Oxford to London Paddington. Many departures per hour, and it takes about 1 hour.

Look up times and prices, and buy tickets on the national rail site: http://www.nationalrail.co.uk/
Tickets are a lot cheaper if bought in advance.

The GWR (Great Western Railway) provides direct service between London Paddington and Oxford. So easy to travel between both cities. The Tube doesn't extend ~60 miles to the city of Oxford.

https://www.gwr.com/

Also two trains an hour from Oxford to London Marylebone. Advance tickets start from around £5 one way on either route.

As noted the London Underground ( tube ) does not extend to Oxford.

Take the train or bus. Bus equals coach in terminology.

Use the aforementioned website for rail options and this for bus options. Your route would be from London Oxford Gloucester Green Station to London Victoria Coach Station www.nationalexpress.com

Train probably the easiest. Then once in London get your Oyster card and use the tube.

There is a tube between Oxford and London though probably not the one you think

https://www.oxfordtube.com/

The direct coach (bus) or “Oxford Tube” was easy, reasonably priced, and convenient when we used it a few years ago.
Have fun!

Coach X90 between Oxford and Victoria Station London takes about 2 hours for 14 GBP return (8 GBP one way), 12 GBP return for seniors and young people.
https://x90.oxfordbus.co.uk/fares/

Which of all the possibilities is 'the best' may largely depend on where your son and family will be living, as well as their costs. The two coach services make stops through the south east quadrant of Oxford, as well as Thornhill Park & Ride. The Marylebone trains as well as starting from the station to the west of the centre stop at Oxford Parkway, with an associated large car park, on the northern edge of the City.

Thank you for everyone's advise. This is going to be quite an adventure for my son's family. Earl

Also expect significantly longer journey times on the coach routes if you are travelling into London during the rush hour.