As far as feeding a solitary fish, it can be difficult especially as a new fish will feel threatened by your presence. Some "dither fish" like otocinclus may make Alejandro feel better.
FYI, a dither fish is a smaller, non-aggressive species that, by acting "normal" assure your solitary fish that it is safe to be out and about.
In the meanwhile, the best way to feed is small, small amounts, one at a time. In the past I have fed a single fish one flake at a time, because if I fed a pinch of flakes, the rest would fall to the bottom and be wasted while the fish concentrated on the one flake it was eating.
With brine shrimp that scatter and can't be fed one at a time, use a sharp knife and cut out very small pieces (a good guideline to start is the size of the Alejandro's eyeball). Feed, and observe. If Alejandro is gobbling, feed more! If he stops pursuing the food vigorously, then he has had enough for the moment.