I would say that there are no 'hard and fast' rules about what works for a profoundly gifted child. Sometimes the personality of the child dictates that the local public school is best. Sometimes the personality of the child dictates that homeschooling is best. Sometimes that 30K a year school is actually a bargain. You have to get to know your child and at age 5 there hasn't been much time to observe how he reacts to 'grouplearn' situations.
Of course if you say that he has already been kicked out of three preschools then I would suggest that you have had lots of experience with how he does in grouplearn situations.
So I don't see any harm in sending him to the local public school for a while and seeing how things go. He may love developing an independent identity from big brother. Often younger siblings are more skilled in navigating the social waters and waiting their turn and do really well in public schools, particularly public schools where the adults are open minded and flexible.
So I would take the advice of the assessor with a tiny grain of salt. After all, how many kids has this particular individual known who are 'beyond the beyond' gifted like your son?
BTW Have you asked your DH to share with you exactly what he is observing that convinces him that this son in particular needs school?
So back to your actual question. I suggest:
1) join a elist for parents who are homeschooling gifted kids, perhaps you have a local one, or from Hoagies
Gifted Homeschoolers Forum (GHF)
A non-profit, all-volunteer organization that works to educate and advocate for gifted homeschooling families. Work includes a well-regarded website with resources and articles from parents and experts, an online community for support and advice, sponsorship of events with nationally known speakers, and a schedule of conferences, lectures and other opportunities...
GT-World
A world full of resources for parents of gifted children. Mailing lists: GT-Families for families of gifted children, GT-Special for families of twice exceptional children, GT-Adult for gifted adults, and GT-Talk for ... heated discussions moved from other GT-World mailing lists
The TAG Project
A support community for Families of the Gifted and Talented, including mailing lists TAGFAM for Families, TAGMAX for home education, and TAGPDQ for "more-than-just-gifted" families
A2Z Homeschool Chat Room hosted homeschool chats on various topics... check the schedule
Facebook Groups and Fan Pages...
Gifted Homeschoolers Forum for parents who are homeschooling their gifted children...
and here's a link to a forum specifically devoted to homeschooling gifted kids
http://wp.me/P2tcDf-LPlease let us know how it's going,
Grinity