My sons English teacher (6th) grade wants them to write a sentence using only subjects? and expand using only subjects ?

By definition, a sentence must have both a subject and a predicate. There are no sentences with just subjects. Somewhere along the way, there has been some miscommunication between your teacher and son on what the assignment is. Perhaps the teacher wants only the subjects changed or expanded. I would suggest you write a note to the teacher asking for more explanation of the assignment.