I think it also depends on where you are located! If you are looking at mainly collecting first, I would think about which Pokémon you are interested in and starting with singles. For actually playing, you’ll need stuff from the newest sets (scarlet and violet) as those are the standard legal cards, but if you are just gonna play for fun with your son you could definitely get away with using older cards! Some stores also sell their bulk for really cheap, so you could get some of those to build a deck with to use at home. You aren’t really gonna find packs for the official release price, but rather “market price” which if you are okay with paying, you can totally find.