My Milian Pere - mount to allow attaching a child seat to a Brompton folding bike - arrived in the mail today. This piece of hardware attaches to the Brompton. How do I attach a saddle to it so that my son can ride in it, which is why I bought it.
Assuming your son will be sitting on a saddle then you need a saddle with a metal clip that interfaces between the rails of the saddle and the post. They are adjustable so the seat can be almost any angle in relation to the seat post. Brompton make one of the clips but you can probably find them cheaper on ebay, or included with some saddles. See images below: