From my understanding people with ADHD often have a ability to fully apply themselves to a application if it has an interest to them,
I commend you on trying to help your brother.
It is not well supported on this forum, but would recommend you look into the Picaxe chip,
it is a micro processor that is very easy to use and will allow your brother to write simple or advanced programs to control the functions of the chip.
You might say its a electronic kit all within a single chip (although there is a range of chips with various abilities of functions) as for a starter i would recommend the small 08M chip.
The picaxe is very well supported with it own forum (search picaxe forum)
This approach will allow many different projects or applications to be done from a single circuit.
As he presently spends many hours at the pc, a few hours constructing a program to run basic functions could be stimulating to his requirements.
To give an insight...... the Picaxe chips was designed for school kids to use, and the rest of us have taken it and driven the wheels off it because it is so easy to use, and so versatile and its fun to push it to its limits.
I am happy to help get you started if you think this is of interest to your brother.
Pete.