So sorry for the suffering your husband is experiencing and you are witnessing. I hate GBM.
Anyway, this is just a shot in the dark, but is it possible that your husband has had a small stroke while on Avastin. See the link to a Pubmed article on stroke and headaches:
http://www.ncbi.nlm.nih.gov/pubmed/20816453
It seems like the NOs would have figured it out if it was, but maybe it is so small the MRI is not picking it up.
Avastin does increase stroke and brain bleeding risk.
I hope someone figures something out to give your husband back some of his life. God bless, Sally