That's so sweet!
Really you can start anywhere if you wish. The DS and 3DS are a great way to start with kids. However, the nintendo switch is more relevant so if you're interested in playing the newest games you can buy that and start with Let's Go Pikachu/Eevee or even wait for Pokemon Sword and Shield. If you find getting the switch a bit pricey, they are releasing a Switch lite and you can always wait for that.
A majority of the pokemon games are on the DS/3DS but if you choose to get a 3DS, you can play all of the DS games on that system. My personal recommendations are Pokemon Heartgold/Soulsilver (each duo of games has some slight differences if you and your son want to mess around with that) and I recommend Pokemon Black and White as well as their sequels. Really it depends how you want to play. For something a bit more easier to step into go with the former, but the latter is definitely a nice follow-up.
There are also 3ds exclusive games as well. Pokemon Ultra Sun and Ultra Moon are good starts as well as Pokemon X and Y. Just keep in mind for Ultra Sun and Ultra Moon they're basically the "updated versions" or Sun and Moon (there is a difference).
Hope this helped a little.
Ducolamia^_^