What diagnostics have you run, other than a physical exam and fecal?
The unfortunate reality is that neoplasia of some sort is the #1 cause of weight loss in geriatric pets. Heart failure, liver disease and endocrine diseases follow closely.
If you dont want to work him up, and want to deal with the weight loss , you can always add something like “maxcal”. Its not the most palliative, but is quite a bit higher in calories. Calories = more weight, regardless where you get them from. Just remember that even with a significant increase in calories, he may or may not gain weight depending on if he is healthy or not.
Im assuming he has had liver values checked regularly while being on phenobarbital?
If you want to look further into why he is losing weight, I would consider an abdominal ultrasound to rule out organ failure (liver, kidney, adrenal, etc), lymphoma, carcinomatosis, hemangiosarcoma, adenocarcinomas etc. Often these are the main concerns in older dogs that do not always show up in bloodwork.
Also, when you said he had no thyroid hormomes…did you look further into that? Sometimes thyroid results can come back altered when there is a thyroid carcinoma (not hypothyroidism).
Hope you find a solution that works out for you and your pup!