The thyroid supplementation may help him.
Seizures can be triggered by a low thyroid level and typically dogs with a low thyroid are lethargic, not eating well, but stay heavy. You can read about thyroid issues here
http://www.marvistavet.com/html/body_hypothyroidism.html
Another possibility, as you are seeing dark stool, is that he has an ulcer or other cause of bleeding in the upper gastro intestinal tract leading to his lethargy, unwillingness to eat, and might have triggered a seizure like episode if he was in a lot of pain or if he lost blood making him suddenly weak.
I'd make sure your vet knows about the dark stools.
An over the counter med you might try for stomach upset would be Pepcid or Tagamet or Zantac.
Be sure to tell the vet when you see them if you use any of these.
You can read about dose amounts (and cautions) scrolling down on the pages here
http://www.petplace.com/drug-library/cimetidine-hcl-tagamet/page1.aspx
http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx
http://www.petplace.com/drug-library/ranitidine-hcl-zantac/page1.aspx
these are also used for ulcer issues.
If that helps then you can see if some bland food and plain pedialyte added to the water bowl will help.
I would suggest trying boiled chicken meat or hamburger with the fat drained off, mixed with really well cooked rice in a ratio of 1/3 meat to 2/3 rice to see if that works. It's not a balanced diet but OK to use for a few meals while working on controlling digestive upset.
However better than home care would be getting back with your vet about the refusal to eat and the dark stools and that would be my top suggestion.
Hope this helps you!
Pet Health Care, Rescue,Train,Breed
30+yrs pet vet care & nursing, rescue, behavior & training, responsible breeding, small animal care