Son has bent his solid rear axel. Can i replace it with a hollow one and reuse the cones and bolts etc?

If it’s long enough and the same thread pitch then it’s fine. Often a hollow axle will be shorter because it’s designed for a quick release rather than nuts

Much thanks.

As above, but not all hubs have the same diameter and thread pitch, IIRC Shimano are the odd one out (e.g. novatec axles might fit a.n. other brand hubs, but won’t fit Shimano).

A hollow one won’t be long enough, as it’s for qr with no nuts

Try velosolo’s solid axle if you need a good quality replacement.

https://www.velosolo.co.uk/shophub.html