The boy’s costume here is known as a “Skeleton Suit” which would have been worn up to age 8 or 9, after which he might be put, a the beginning of the period, into breeches and waistcoat, with a relaxed long jacket. Later in the period, the boy of this age would wear a short, tailless jacket and long trousers, [the Eton suit], and this style continued throughout the 19th century and beyond. Boys were put into the skeleton suit at about 3 or 4 years old; before this they wore a frock, which sometimes makes for difficulty in distinguishing them from theirs sisters in portraits.
The skeleton suit was usually made of heavy cotton or linen, which were both practical washing fabrics. Blue was a favorite colour, but examples in pea-green and occasionally scarlet or mustard are also documented. Charles Dickens described, in 1838, some time after they ceased to be worn, “A skeleton suit, one of those straight blue cloth cases in which small boys used to be confined in ingenious contrivance for displaying the symmetry of a boys figure by fastening him into a very tight jacket and then buttoning his trousers over it so as to give his legs the appearance of being hooked just under the armpits.”