Having already taken my sons down the same route I would suggest the following.
A good starter gun is the 28 bore on 14 or 17gr cartridges yes I know they are expensive but that's the way it is.
Teach safe handling, my sons can withdraw a shotgun from it's slip and it's fully opened by the time you see it, I can elaborate if required.
Don't see the point in .22 rimfire as a firearms license will be far harder for him to get, in theory you can have a shotgun certificate at a very very young age.
If you shoot a 12 bore I would recommend not having a 20 bore in the house as there is the possability of dropping a 20 bore down the barrel and then loading a 12 bore behind it one bang later and minus a hand you will understand the danger. If it can go wrong then it will go wrong.
Gun safety is quite easy to teach and go along with the clay instructor, make sure the gun is always open and never has any cartridges in until on the stand and ready to fire. Make sure the gun is put back in the slip between stands in the open position and closed at he slips it in.
The next gun up would be a 12 bore using 21 gr cartridges, there's not much recoil.
By all means post any questions.