I understand your concern.
Well as you describe,his current symptoms are not suggesting cancer.
Low red blood counts can have various causes, including vitamin deficiencies, bleeding, and anemia.
Normal range 4.7 to 6.1 cells/mcL
If his red blood count is abnormally low ,then his doctor may do other blood tests or take a sample of bone marrow to find out why.
With just said low red blood count signal cancer without knowing the exact value is not correct and definite.
I recommend to see his regular doctor again and a clinical correlation of his blood work findings should be considered.
He should limit his fish oil consumption as long term use of it can decrease immunity and increase the risk of bleeding and infection.
You should give him Oral rehydration solution.
Hope this helps.
Use reply to let me know if you have further queries.
Please to not forget a positive rating.