Hey Cod,
I'm cabin crew and also a parent with 2 similar age young ones.
The best advice I would give is buy a seat for your son (even though he can go free on your lap). Don't rely on the flight being not full because having that extra seat for the young one will make all the difference on a full flight.
This also means you will have the option to bring along an approved car seat, which he may be more comfortable in and consequently he may sleep better (and more importantly he will be much safer).
I try to travel on a flight that leaves around his bedtime or a couple of hours earlier.
I usually try to board the aircraft towards the end of boarding, but this will depend on how organised you are. The reason for this is it lessens the time you have to keep them in one place (until take off).
Don't bring too many toys, just one or two of his favorites and a book or two that he likes, and delay giving them until they are bored with everything else that is already in front of them.
That's a start anyway,
Good luck.