Congrats on your adoption!
Don't worry about doing it "wrong". Any extra stimulation he gets is going to be beneficial. I am so busy, especially in the winter that very often I am not able to be nearly as consistent as I would like, however I am still so thrilled with K's progress.
If he already loves books, that is a big help, & a good attention span in wonderful. Just make sure you quit when he is still enjoying it so that learning stays fun. When you read to him, run your finger under the words to help bring his attention to the text as well as to help him become familiar with the individual words.
Flashcards are wonderful for a visual learner, which most kids with DS are, so of course I would start there. Have you tried Little Reader yet? Their flashcards have wonderful, clear concise pictures & many are free. The above link sends you to the details of their special needs discount which they are VERY generous with.
As for the amount of material to cover, I don't think there is an exact right or wrong. You have to consider his interests, enjoyment & as you mentioned, attention span. In the beginning, when K was only about 6 months old, I did sets of 10 or 12 flashcards (word & pic) & showed them about 3 times a day, but as K seemed to learn faster, I gradually moved up to an average of 45 words per week.
If you haven't been to the Unlimited Potential forum yet, this is a great resources thread. There are lots of resources mentioned that will be very helpful.
Have you checked out my important posts page yet? It's a work in progress, but I'm trying to compile a section on teaching reading tips.
Lastly, have very high expectations for your little boy. Give him lots of input, lots of exercise, explain things, & most of all enjoy him!
If you want anymore specifics, please ask away:)