There are many stainless steels. They are formulated/discovered to have resistance to various corrosive agents I think it is a big help to keep in mind that these steels "stain less" and that they are not "without stain". The right ss for the applications can serve very well for long periods of time.

I don't know what ss is used for bike cables and I would guess the various manufacturers use different ones. But a bike parked it the rain is not a severe environment. Even the lesser stainless's should do just fine. Maybe bleed a little rust after a year or more but unless cosmetics matter, not a big deal.

I still would occasionally lube the cable entrances and exits with grease or oil just like I would if they were not stainless.

Bigger issue for you/your son - with ss cable, it isn't obvious when the wire is getting tired so it is easy to use the cable too long and break one.

Ben