I would probably start with stuff that tastes yummy.
Much easier to train your bird to enjoy syringe feedings that way and you can often mask bitter or bad tasting medication by mixing with a favorite flavor. He might even like the taste of the medicine or at least tolerate it, so there would be no need to subject him to bitter or disgusting fake "medicine" as a training practice.
The main goal should be teaching him that the syringe brings good things, like water, juice, nectar, tea, or hand-feeding food. Then, you will have a much easier job when it comes time to offer an oral medicine, since your bird won't fear the syringe or the handling steps necessary to give the medicine to him.
As you work with your bird, you will probably learn what flavors he likes and which ones he hates, so if you feel like training him to accept bad-tasting liquids, you can just use his least favorite juice flavor. But my recommendation would be to focus on the positive so he builds strong good associations with the syringe feedings and then try new flavors occasionally to expand his palette.
