Hi
Mum experienced a great deal of cognitive impairment (similar to what you describe), but probably more pronounced, in the last couple of months of her battle. It started quite inoccuously, but proceeded very quickly to quite significant impairment. She did have brain mets and treatment for them about six months prior. Whether the symptoms were due to the lesions, edema, the treatment, something else altogether, or just the disease process, we don't know. Most of her blood work was OK.
Some of the things (other than brain lesions) that can cause cognitive changes include blood sugar, serum calcium levels, dehydration, steroid use, side-effects of treatments....the list goes on.
If his brain scan came back clear, I would take heart from that, but I would probably also consider asking for another one. Mum's lesions were apparent on CAT, but an MRI is the better diagnostic tool.
If your husband is talking about going back to work, then his level of impairment is nothing like as significant as Mum's, and I would think it possible that there might be some other (hopefully less serious!) cause for his condition. I think that cognitive symptoms due to brain lesions tend to progress quickly and significantly, amd would be obvious to others besides yourself. But an MRI would certainly clear up that issue.
All the best,
Karen