The immigration people at the airport will disturb him if he doesn't have the yellow fever card when he is leaving. They do immunize people at the Accra airport for a fee, if you require it. According to WHO the yellow fever immunization is now for life. I am not allowed to post links but the information can be found on WHO's website.
"17 MAY 2013 | GENEVA - The yellow fever ‘booster’ vaccination given ten years after the initial vaccination is not necessary, according to WHO. An article published in WHO’s Weekly Epidemiological Record (WER) reveals that the Organization’s Strategic Advisory Group of Experts on immunization (SAGE) has reviewed the latest evidence and concluded that a single dose of vaccination is sufficient to confer life-long immunity against yellow fever disease.
Since yellow fever vaccination began in the 1930s, only 12 known cases of yellow fever post-vaccination have been identified, after 600 million doses have been dispensed. Evidence showed that among this small number of “vaccine failures”, all cases developed the disease within five years of vaccination. This demonstrates that immunity does not decrease with time."
