Some children are just more sensitive to the discomforts of plane travel, although all babies have some distress. Some of the problems your son has will disappear, I am sure, as he grows up.
On the day of travel, you might make the child play actively all day so that he is ready to drop off to sleep when you board. A warm drink of milk just before sleeping time is also generally sleep-inducing. If the child has a favourite toy or other object that he holds in his hands before sleeping, be sure to pack it in your hand baggage. In any case, do your best to keep the child quiet and resting, but don't get embarrassed if he does not comply. Everyone in the plane was once a baby and at least half of them must be having a young one at home. Talk to other women you know, who have to take long plane trips. They might also have some useful suggestions.