Chocolatelovinggran, it's perfectly possible to buy a tortoise, but it has to be from a reputable breeder in this country that will provide a CITES certificate. Times have moved on from when I bought mine for my 5th birthday from a fruit and veg stall in the local market for half a crown! It has been illegal to import certain species of tortoise for quite some time and I fully support that, as so many tortoises were plundered from the wild in years gone by and are now endangered as a result. Mine was probably one of those, but we know better now. The most suitable ones for out climate to buy from UK breeders are Hermanns, Mediterranean Spur-Thighed (which is what I have) and Horsefields (although they are notorious diggers!) They are also not cheap to buy any more!
Baby tortoises need a lot of care (as do older ones, but different care needs). If your grandson is keen to own a tortoise I would encourage him to do his research as any baby tortoise would need an indoor tortoise table with special heat lamps and careful temperature monitoring. This takes up quite a bit of space. Plus tortoises don't just eat greens and lettuce, as most people believe. The best diet is natural weeds/flowers but this again needs careful research. They are not "easy" pets, and once they get to hibernation age that again needs careful management. Oh, and just to dispel the myth, tortoise DO drink and do need water available at all times.
Having said all that, they are the most fascinating creatures and well worth the effort!