A: Dear Parent:
Wait until he is interested in reading with you.
Ordinary loving conversation is the best way for increasing the vocabulary of an 18 month old. No parent need to concentrate on this--the everyday sharing of experiences and observations and affection is the way that parents and toddlers grow together to be good listeners and conversationalists.This is a natural process.
Reading stories to a tot is great fun if the tot is interested. But listening to the parent read is just not on the radar for many children of this age. Ten it's not accomplishing anything, as you have observed. Your little guy is action oriented at this time--I'd go with your intuition about his level of interest and follow his lead. I'd put the books aside and try again in a few weeks or months. There will be plenty of time for reading together later.
Being alert to the child's own developmental level is a good guide to when to introduce activities. You can guide your active little boy to bed and then tell him a made-up story that is five sentences in length. Or let him tell you stuff. A little chat now can be a precious experience. At his age, he is learning from people more than books.