In your opinion is this suitable, or should I wait until he is a little older and give him a 20-bore?
As long as the lad isn?t very small I can?t see any reason why he shouldn?t start with the smaller 28-bore so long as it is light and properly balanced.
Don?t forget that the stock will doubtless need shortening and that the off cut wood will need to be stored safely should the stock need to be lengthened as he grows older.
A 28-bore is more than capable of dusting clays at most reasonable ranges so don?t worry that your boy will be ?undergunned?.
The important thing is to protect him from recoil by choosing a light, soft, cartridge and to introduce him to easy targets so that his confidence gets a boost and he enjoys the game from the start.