Robin, if you plan to adopt another kitten, do it at once, and try for one of the same age. Many people tell how happily their kitten has accepted another–but I never had that good fortune. Mine have been somewhat tolerant of one another, but never cuddly bosom-buddies.
If you kitten seems happy–not sad, not meowing mournfully, not searching for his missing brother, then don’t worry. In my experience, only siblings seem to take to one another, and never have seemed sad when, one by one, litter-mates left for their forever homes.
You said that you had intended to adopt only one. The advantages of just one are that you’ll always know how much he eats, drinks, uses the sandbox; such things are rather difficult to determine when there are more than one, and that can make keeping a cat healthy just a bit more challenging. (There’s also only the fur from one…)
Keep your little fur person supplied with toys– simple things such as an empty thread spool, a wadded-hard ball of aluminum foil, a marble, the plastic ring torn off when a bottle of milk is opened; store-bought toys are fine, but not essential–a bed or two of his own here and there, access to windows so he can watch the world go by, mostly hard, dry food so his teeth will stay healthy, unless your vet advises otherwise due to a medical condition. Have him neutered as soon the vet agrees, so he will not develop the male habit of sprayng when in heat. Then just give him all of the attention and affection you can; talk to him and play with him; brush him and stroke him.
He’ll be fine, and so will you, unless he AND YOU truly want another cat, forever.