Answer:His hippocampus has been damaged.
Step-by-step explanation:
Hippocampus is part of the brain that is located in the middle section of the brain known as the temporal lobe.
This section is responsible for leaning, acquiring information or memory and for regulating feelings and reactions.
When this part of the brain is damaged during accident or when it is affected due to Alzheimer's diseases, it stops functioning which can lead to a person's inability to keep short-term memory .
This means the client above during a sever concussion he probably hurt this part of the brain which is now affecting his ability to keep short-term memory and it is also affecting his feelings causing him to be depressed.