The only train simulator on console that I know of is Train Sim World (TSW), there are 5 iterations. The latest is TSW 5. The routes and traction are limited in number compared to PC as it only started in 2018.
Just had a look at pricing. The official Xbox website has TSW 5 special edition at a special price of £26.24 discounted from a full price of £104.99.
The routes that come with this edition are:
UK routes:
WCML: London - Milton Keynes with Class 390, Class 350 and Class 377
London Underground Bakerloo line
London Overground Suffragette line with Class 710
Plus these foreign routes mainly USA/Germany:
San Bernardino Line
Frankfurt - Fulda
Kassel - Würzburg
Cajon Pass
Bremen - Oldenburg
NEC: New York - Trenton
That‘s a good price considering the standard edition is priced at £39.99 on the Xbox website and you don't get anywhere near the amount of routes as in the special edition.
If you want more routes (DLCs), they are usually priced ~£25 with special discounts sometimes available. From what you describe, he may like the:
Spirit of Steam DLC (Crewe - Liverpool set in 1958 with 2 types of steam trains): £24.99
West Somerset Railway (with Class 47 and Class 09): £19.99
Once you've bought the base game TSW 5 and have downloaded it, once you open the game there is an area to see DLCs that are available to purchase which you and your son can browse to see what tickles his fancy and what routes are on discount.
