Most children who kill themselves have attempted suicide before. Also, many children who attempt suicide later kill themselves. A suicide attempt is always serious no matter how trivial it may seem at the time and no matter how non-lethal the means used. It is not necessarily true your child knew the method he used would not kill him.
Suicide is a process and not a single act. It is seldom just something that happens. A child thinks about it for a long time. He adds up his losses, disappointments, humiliations, times he does not fit in, and when he thinks he does not belong. These all build up to feelings of not being in control, hopelessness, and being unable to make things better. Any child who attempts suicide always needs professional counseling and special help to understand these intense feelings and to learn new ways to manage them. He also needs his parents, needs to talk with them about his fears and frustrations, his anger and emptiness.
(Note) About a third of the time when children succeed in killing themselves, there are no signs sufficient to suggest suicide, that could have or should have been noticed by parents and others in the child's life. It is as if it just happened with no warning or prior indication.