I use these 2. Camelina oil and kelp powder.
The oil has a pour cap on it. You probably could get fish oil in a bottle. 
I have a picky eater too but ever since I started grinding everything up in the Ninja, he eats the whole thing then licks the bowl. Dogs are funny with what they like. The other dogs eat absolutely everything but Max turns his nose up at almost everything except meat. 
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