Something to call someone who doesn't have a say in the choices that affect him the most?
Think 'a major stakeholder who has no voting rights,' ha!
I would say powerless. This can relate to both single or ongoing situations.
A child is powerless if parents decide to get move house/send him to a different school/ get divorced (depending on the style of parenting of course).
People are powerless in the face of governments which take no regard for their needs or views and impose unwanted regulations.
Depending on the situation you could say someone was at the mercy of, or subject to the whim of.
The word dependent seems to fit nicely.
Defined as: determined or conditioned by another; decided or controlled by something else; needing someone or something else for support, help, etc.
Children are commonly referred to as dependents. Generally dependents have all their decisions made by the parents/guardians. The synonym - independent - is descriptive of someone who makes all of their own choices and decisions.
You could say the person "lacks agency" -- the ability to act, or to exercise that ability.
In this connection, you might find this other current ELA Question on the words 'agent' and 'agency' of some relevance or use.
(Just to be clear, I am NOT suggesting that either of these questions duplicates the other.)
Consider the term puppet
one whose acts are controlled by an outside force or influence - a puppet ruler
Often the term is used to suggest someone who appears to be in control, even when they are not. Also, it is not specifically limited to control of choices that affect him the most, although the term is usually used to describe important issues.
While it has many more implications, the term slave also refers to someone whose lives are fully controlled by others. One definition other than those that assume actual human ownership is
a person who is excessively dependent upon or controlled by something: the poorest people of the world are slaves to the banks