My son (21 months) also cannot sit down to read cards / story books. However, he can sit down and listen to songs and music during playgroup time. May be you might find another playgroup which he is interested in?
don't worry. Some kids tend to ignore the instructor and other kids in the first few classes. The kids may run around or ignore some of the activities in the class. But don't stress out. They will get better after several classes. Give your child some time to feel comfortable going to playgroup, since this is a new thing to him.