Hi, I had gone through your question and understand your concerns.
Yes , flu vaccines like Vaxigrip, Agripal  are available to vaccinate your baby.
Flu vaccines cause 
antibodies to develop in the body about two weeks after 
vaccination , so it is recommended to get vaccine  at least 2 weeks before travel , and get a booster shot after 1-2 months of Ist dose , thereafter a single shot every year as long as  protection required.
 You should get him vaccinated before travel .
Regarding degree of protection it is not straight forward as it depends on a number of factor, like body's response to vaccine , similarity between circulating strains of 
influenza virus, and vaccine strain , roughly there is 70-75 percent reduction in hospitalization due to flu in vaccinated individuals.
Moreover , all persons in family getting vaccinated in family is an additional protection for child.
Hope this answers your question. If you have additional questions then please do not hesitate in writing to us.
Wishing  you and your child good health.