Paul, I am sorry that you met up with some of our less desirable members > >! Believe me, they are in the minority.
As for your dovecote, I think its a great idea and it is how I first started keeping my pigeons. Fantail doves are not really suitable for a dovecote they are best suited to an aviary as they can't escape predators readily. What you could keep though are garden fantails - they are similar but don't have such big fans. They make lovely pets and easily tame so that they fly to you and waddle after you around the garden.
They are good at escaping predators apart from birds of prey such as the Sparrowhawk, it would be devastating for your son to see one of his birds caught, plucked and eaten alive!
The downside of keeping the birds outside is the droppings....you would not believe how waste one pigeon produces! If you can keep the birds on the dovecote then it isnt a problem. Mine insist on perching on our balcony and also next doors! Do post again and ask any questions you have, we will do our best to help you.