Bill, age 11, noticed that he has some pubic hair. according to the usual sequence of puberty, his next pubertal event will probably be

The fact that Bill is 11 years old, means that he is in puberty, which is the process of physical changes through which a child's body matures into an adult
Bill, age 11, noticed that he has some pubic hair.
The sequence is:
1) growth of testes, 2) first pubic hairs, 3) enlargement of penis, 4) first ejaculation of seminal fluid (spermarche), 5) height spurt, 6) final pubic-hair growth. 
According to the usual sequence of puberty, his next pubertal event will probably be penis growth.