If grown child moves back home, what is his responsiblity to help?

Depends. You often get the kid you build.If they move back expecting to freeload,it's time for a review of terms.If they have a job, they give up a percentage of the proceeds to support the family.If no job,they go out every day to find one.You give them life,you nurture them to adulthood and you let them know what the expectation is long before they become adults, so this is not an afterthought.
BACK HOME. Conservative families like the arrangement of getting together again if parents are old and they help out in the house. They can save their money if parents finances are okay.The reciprocal relationship gives love and comfort to everybody.