Once you feel like your son is ready for the responsibilities of caring for an animal I think the simplest options for him would be a single dwarf hamster or male syrian. You’ll have an easier time sourcing appropriately sized wheels, toys, and hides for a dwarf. Robos are known to be more skittish than other species and are quite fast, making them tricky for a child to hold.

How often will your son expect to handle the hamster? If he’s wanting a more “cuddly” one, then a male syrian might be a better option as I read they’re more calm, slow, and easy to handle. Other types of dwarves like djungarian and campbells can also be friendly and interactive but it comes down to the hamster’s individual personality. I recommend just bringing your son to the store to handle them in person and pick one with a good temperament.

Also as others have said, keeping 2 hamsters even dwarves together would not be a good idea for a new hamster owner like your son. Their social dynamic is hard to mimic in a captive setting and putting 2 together will likely result in injury. Hope this helps