Hello,
Not sure if they are RC controlled cranes in the traditional sense, in that they are wired, but can be controlled electronically (normal digital controllers like CS, software, some come with modern style control pads/joysticks like Xbox) or via old style analog controls (push button), please see these items
Marklin items (model number)
Marklin crane 7051 - This model was originally analog controlled from small electronic box (push button), can be upgraded for digital control through digital controller and there are articles on this Forum on this subject. IIRC there was also an early electronic version issued as well. See link
linkMarklin crane 76515- Digital crane - see this
linkGantry Crane 76501 - Digital crane see this
linkLarge Coaling station with comes with a coal loading crane item 76510 - Digital crane - see this
linkMarklin have also made models of railway cranes that can be used on the layout and operated remotely using digital such as the Goliath 49952 - see this
linkThe Goliath can be seen in operation here;
ROCOPortal Crane Set 41290 - digital crane - see this
linkIIRC ROCO also made working railway cranes for the layout like Marklin such as the 46902 (but ROCO is usually 2 rail and will not work on Marklin 3 rail so check before you buy)
BRAWAContainer crane 1162 - An old and analog model, hard to find - see this
linkHeljanContainer Crane - Digital container crane with many functions - see this
linkI would suggest you do some internet research based upon the model codes given above, many items are sadly discontinued but often available, spares can be hard to obtain, some are easier than others to get working and some are quite expensive. Any questions I am sure someone in this Forum can help
Good luck, and let us know what you choose
BR
PS: Why not check out the Marklin Club UK pages on the Forum, we are not yet a formal club, but we do meet a couple of times a year, and are involved in exhibitions from time to time. I am sure your son (like mine) will enjoy being involved in these events.