My suggestion is that you search for homeschool groups in your area. You may be able to find it by googling homeschool group and the name of your area. Or, try
http://homeschooling.gomilpitas.com/weblinks/support.htm Also, look at yahoogroups as many are listed there.
If you subscribe to a few local homeschool lists you will have a much better idea what is available as far as co-ops, field trips, classes, etc. In many areas it would easily be possible to be out of the house at homeschool activities every single day. Having a plan for activities may help convince dh and the therapist.
Some families find they start with something like k-12 but then find it is similar to public school and lacking in the degree of flexibility their gifted kids need. At that point they are more confident in their abilities to take on homeschooling planning.