Observe real-life boys as models if possible. They can be your own boys, a friend's boys or pictures in a catalog. This can help you see how a boy dresses and moves, and it can make it easier to fine-tune the proportion of the body parts.
Draw your little boy's head larger than his body, in a rounded shape. The head-to-body ratio for manga drawings of small children can be one to two.
Add eyes that are large and round. Set them low down on the face and omit eyelashes. This lends an innocent look to your character.
Create tiny circles for the nose and mouth.
Make some short hair, being sure to show the boy's ears. This will alert the reader to the fact that he is looking at a male character.
Draw a torso and legs that are narrow through the hips and bottom. Manga boys are not as fleshed out as girls. A boy's feet will be bigger in relation to his body as well.
Add simple clothing, such as shorts and a t-shirt. Shorts, rather than pants, will show the character as being very young.
Mature your male character by drawing his face with a more angular look. His eyes can be smaller and more almond shaped as you age him. Boys in manga tend not to have eyelashes, no matter what their ages.
Make the nose more defined to show an older boy. His mouth can be bigger too.
Move the hairline of your boy farther up on his head, and create a more mature hairstyle.
Elongate the boy's body, so that he is still thin and wiry looking, but taller. The head-to-body ratio can approach one to eight, depending on the age of the character.
Draw feet that look big compared to the rest of his body. This foot-size discrepancy exists in real boys as well.
Dress your manga boy with clothing appropriate to his age. Long pants can be added for an older look.