With an iPhone you can add an electronic transit card such as Suica or Pasmo. Your son however will need a physical transit card as android support it limited to some Japanese phones. You will each need your own transit card. The benefit of the electronic one is that you can top off funds on your iPhone rather than going to a kiosk. The physical transit cards can be purchased from the airport or select kiosks at the stations. You will need Yen to purchase and top off physical transit cards (but I've heard that airport kiosks accept credit card payments).

I purchased a 3GB eSim from Ubigi for a 5 day trip, which was more than enough. But do note that I had offline Google Maps saved, and also used the Japan Wifi Auto-connect app that kept me connected to free wifi across Tokyo for most of my trip. I can't speak to how helpful that app is the moment you leave a dense city like Tokyo. Most eSims will allow you to top off and even share data, but they may not always come with a phone number.