I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site. Yes, neurologic disease but with a few different etiologies need to be considered. Diet is important to review if only because vitamine E deficiency can cause encephalomalacia ("brain softening"). What has his diet consisted of, please? Seeds should compose less than 20% of his diet. A diet of mainly seed and nuts has excessive fat, carbohydrates, and phosphorus; marginal protein; adequate vitamin E, and are deficient in amino acids, calcium, available phosphorus, sodium, manganese, zinc, iron, vitamins A, D3 (necessary for efficient absorption of calcium), K, and B12, riboflavin, pantothenic acid, choline, and available niacin. Ideally, a balanced pelleted diet such as can be found here: www.harrisonsbirdfoods.com or here: www.lafeber.com/pet-birds should be fed as well as hard boiled egg yolk, pancakes and cornbread, the tops of fresh greens, dairy products such as yogurt and cheese, fresh fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat.
Toxicosis such as with lead can cause his symptoms. Idiopathic (unknown cause) epilepsy needs to be considered if his losing his balance and ending up on his back represent partial seizures (previously called petit mal).
Infection in the central nervous system isn't likely because such an infection would have led to his demise before a month or more passed. Strokes which are relatively common in birds aren't common at 18 months of age. Trauma, however, could potentially induce a stroke.
We're necessarily constrained by how much we can do testing-wise and treament-wise in a canary. The antibiotic and anti-inflammatory drug are reasonable. I would consider an anticonvulsive drug if seizures are likely. Has his vet seen your canary lose his balance and end up on his back?