My 7mo ds cuzen act like a newborn, he doesn't develop almost anything yet. He can't sit. Doc said he maybe half-blind. Does that slow his development?

Being blind does not. Slow development unless it is part of a syndrome..7 months old with downs syndrome will definitely be way behind in developmental milestones in all aspects due to hypotonia.Need to start physical therapy as well as speech and occupational therapy as soon as possible!
Many factors. A infant's development can be slow for many reasons. If he was born premature or had problems at birth, his development may be delayed. Vision problems can occur with or without other problems of development. If the parents have other concerns, a pediatrician who specializes in behavior and development could examine the baby and discuss options with the parents.