The simple answer is that he isn't dressed like a girl, he's dressed like a child. Up until the early 20th century children were dressed alike up until a certain age range (my grandfather included). Boys usually moved out of their skirted garments around age 4-6, girls around 12-14. For boys it was dependent on potty training and manual dexterity. Unlike today where children's clothing has elastic, velcro, and at most a zipper, clothing was most often fastened by way of buttons for men. A boy moving into a pair of breeches in the 17th or 18th century will likely have a minimum of 12 buttons to do or un-do. A skirted garment is just easier to deal with changing the clout or letting the child be more independent at an early age. There was even argument in the 18th century for leaving boys in their gowns as long as possible because they wouldn't have any leg restriction. Thus running and jumping would create stronger, healthier legs.

There are some great portraits that show the transition well. This portrait by William Hoare has three brothers, likely just past ages 1, 4, and 6. The youngest is already in shoes made for walking and stays, so he's upright and moving. And while it's not always as easy as this portrait by John Francis Rigaud of his children to tell if the child is male or female, there are usually subtle differences. Boys hair is generally kept above shoulder-length or even very short, their clothing is usually very simple in decoration, sometimes even has masculine trimming like reveres, their props tend to be things like dogs or games rather than flower baskets, etc. Fruit is even pretty common, though in your image I assume the oranges are there for other reasons. There is a dog, however, and the hat is a man's style despite being over a child's cap. To compare here is a portrait of a six-year old girl from nearly the same time. She's carrying flowers, her hair is dressed in a simplified woman's style, and her gown style is cut ever so slightly more like an adult woman's as well.