An inflatable kayak is one thing, but I strongly recommend going no where near one of the Toy inflatable boats linked above.
Even the 2.3m Seago linked above is not what you are looking for. That’s coming from someone who uses a seago every week to get out to my boat’s mooring as a tender and has one sat beside me right now.
They are miserable to row any further than a few hundred meters even with no waves / wind to contend with. Add in wind and a chop and you will get very tired, very quickly.
I would recommend seeking out a local sailing club, they almost all have open days where you can go along, chat to members, and get a shot of some boats.
It will provide you with a base knowledge from which you can decide where you want to take it. I too would recommend joining a sailing club. Many / most have boats you can use for a very small fee and will be, on the whole, well mantained. If you and the boy take to it then go get a mirror / Graduate / GP14 and only sailing it when there is rescue cover. At least for in initial year or two.
I have been sailing all my life and have been on and around boats since before I could walk. It’s a great activity to do, with so many possibilities from pottering about of a Sunday morning, picnics on the beach, some friendly racing to even competitive top flight racing.
However if you don’t know what your doing, things can go wrong very quickly and you will be putting your life in danger.
It is however great fun! Please don’t let me put you off, you should definitely give it go, but please don’t buy a toy boat and put yourself in danger.