Splendid new, Vivienne!
Give him all the encouragement that you can.
If he wants to make a working steam loco, or even a battery electric one, he will need to learn some new skills.
Much as Model Engineering is fascinating hobby, it can be quite expensive, depending on what is desired.
He wants to start with something simple, so that he acquires the new skills and the confidence to use them.
A Meccano set is good starting point, not only will your son become adapt at assembling things (manual skills ) but he will almost inadvertently learn some maths, mechanics, about structures. (pulley ratios, gear ratios, stiffness of devices etc ) and a bit about electrics. All will stand him in good stead in his future life.
As already said, a good starting point might be assembling an engine, as opposed to a loco, from a machined kit of parts. Stuart can supply in three forms, castings for machining, pre machined parts to be assembled, and a complete engine. The price increases by a actor of about five for each stage. Several years ago, a set of castings for a small vertical single cylinder steam engine (Not a locomotive ) was about £65, the pre machined kit was about £300 more, as I recall. Things like the nuts and bolts to complete assembly would need to be obtained locally.
If the budget would run to a Chines mini lathe (secondhand or new ) plus other tools (measuring equipment etc ) would be an excellent starting point for later life. But you are talking thousands of pounds even at a basic level.
He would need to learn Safety, even hand tools could inflict a serious wound, powered machinery even more so!
The a start can be made on learning to use the machine, initially by just cutting metal, and then by making simple things. AS in all things, it is essential to learn to walk before entering for the Olympics!
If the budget won't run to that, as happier times return, find a local Model Engineering Society (Whereabouts are you? Look on Google ) Hopefully, it would welcome a new young member. Some, like Hereford SME, set aside facilities for their younger members, and provide tuition, to encourage youth. (I am not a HSME member, being based in East Anglia , but have seen their facilities and some of what the younger members do )
Possibly someone would be prepared to have your son in their workshop to watch, learn, and help
The country needs Engineers, so the older ones should encourage and mentor new, younger, members.
Hope that this helps a little..
Howard