We feel this will not help him in any way to develop his much needed social skills as well as keep him up to his peers in school grades.
Have you done any reading on homeschooling? It appears you have many misconceptions about children who are homeschooled. Linda Dobson's The Homeschooling Book of Answers is an excellent book that will help put your fears to rest.
The idea that homeschooled children have socialization problems is a fallacy. Homeschooled children interact with their family, their peers, and the community on a daily basis. They are free to learn at their own pace and study those subjects that interest them for as long as they like. They grow and develop according to their own inner timetable, rather than being ruled by peer pressure and outside influences. Studies have shown homeschooled children's social skills to be at or above their schooled peers. In addition, homeschooled children consistently outperform traditionally schooled children on standardized tests.
Your grandson learned firsthand that the school environment did not provide the socialization he needed. He is not alone. Many children, for a variety of reasons, do not adapt well to a school setting. I have seen this with the families in my homeschool group. There are six boys and one girl who all did poorly in school. They were labeled, put in specialized classes, and, in three cases, medicated. After three months of homeschooling, each child showed remarkable changes. Within a year, they no longer needed medication, and were performing at or above grade level. These aren't isolated cases -- I hear similar stories from grateful parents around the country every day. These parents are grateful they took the first step and chose to homeschool -- their only regret is that they didn't do it sooner!
Your grandson's father is very courageous to decide to try homeschooling. Why not support him in his efforts and see what happens? Remember, Joseph always has the option to return to school at a later date. At six years old, Joseph needs to regain his self-esteem and discover that learning can be fun. I wouldn't worry about grades or memorization of facts right now. Good luck!