Tablet or iPad?

If you want a tablet, it doesn't really matter whether you get android or Apple. In that case, you would need to get some way of connecting a card reader or a camera so best to get a tablet with a USB-c port. If you go Lightroom, you have to go with the cloud version of Lightroom on tablets. Note that if you go this route, you need to pay ~$10 each month for the subscription to the Lightroom software and the cloud storage.

An alternative is not to get a tablet but a cheap laptop either mac os x or windows based and install the full version of Lightroom which is called Classic and doesn't rely on cloud storage. It has a more steep learning curve but is also much more powerful. Classic costs ~$10/month but comes with a subscription to Photoshop too but less cloud storage than the Lightroom cloud plan.

Last, you might initially be best off with not getting Lightroom until you know how serious your son is. You can just use the built-in software on an iPad (Photos) or Mac Book to import and edit raw files from any DSLR. On android and windows there is similar built-in or low cost editing software. Lightroom is great but can be overkill.