Ok so does he have a bottle or wear nappies? If so, put them at the top of the list!! Nappies x how many a day + 2 spare is usually my rule and 1 in hand luggage.
Muzzies/comforter x several!
Undies
Shorts x 5
T-Shirts x 5
Cardy/hoody x 1
Swimmies x 2/3
Pjs x 3
Shoes - 1 pair sandals, 1 pair comfy shoes, 1 pair smart shoes
2/3 evening outfits. My DD usually ended up in her pjs at that age straight after dinner so nice evening outfits were pointless.
Arm bands.
Toilettries - toothbrush, toothpaste, shampoo/conditioner, baby bath (double up as wash), Sudocream, sun lotion, Piriton, Calpol sachets.
Suncreams in sprays and covers all in one application, toddler bug repellent.
Thermometer, nai clippers
Hair brush, detangler spray
Milton wipes to take out with us
Hand gel.
Small cutlery and toddler cup.
Hand Luggage
Small blanket (useful for the plane and for comfort)
Drink (when through security), chocolate buttons or alternative that he can suck on the plane.
A few LITTLE things wrapped up to open on the journey or incase you get delayed.
Calpol sachets in a bag and spoon.
Carrier bag with tissues (for sickness)
Wet wipes
Spare set of clothes
Ipod?
Playdough, Fuzzy felt, books, crayons and paper.
Snacks - Pringles, apple, raisins, YoYo bears etc
I'll have a think about what else he'll need based on what we've taken in the past.