What can I do to increase my baby's appetite?

Not much. I agree with dr.Berkowitz that older babies can focus too much on one item, like cow milk which may reduce their food seeking behavior. Most kids are prewired to seek as they need with spurts of growth hormone leading to noticable bursts of eating followed by days of nibbling. If baby is growing normally, with good patterns of sleep and robust play, i would sit back & enjoy the show.
Not much. There is no potion to increasing a baby's appetite, in some cases in which a baby drinks excessively they will not eat an adequate amount of solid food. In those cases we reduce the amount of liquids.