We have a 2 year old daughter, and we have done a number of flights with her since she was born. At 4 months, 5 months and 8 months old, she flew long haul from UK-Aus-UK-Aus, and recently did some domestic flights from Sydney.
The only time we have given her Panadol or any other medication during our flights is when she has been teething as the take off/landing hurts her ears.
I tend to take a couple of drinks, cookies, sandwiches, and a couple of Jelly Lollipops for her to suck on during the take off and landing, and that helps.
With regards to amusing your little boy, some story/picture books, colouring books and a favourite teddy is often useful, as well as his security blanket (if he has one.....our daughter's is her sheepskin, so that goes everywhere with us.) Try not to take a musical or noisy toy if you are on a longhaul flight, as that can get abit much in the confined space of the aircraft for that long.
I hope this helps.