Make sure to take something he will like to suck or chew on for the plane, and have a couple of new little toys he has never seen before...and plenty of snacks.
The regular stuff, of course (diapers, etc.), plus bathing suit, t-shirt, large towels, sunscreen, wide brimmed hat, a small shovel and baby powder (magically gets sand off!) We have a beach bag with too many things to name for more local travel
I recommend those sunscreen sticks, they make it a lot easier to apply to the face w/o getting any in the eyes :)
I said a shovel because when we took my daughter to the beach at that age, she was scared of the water and only wanted to move the sand back and forth with a shovel.
The only special thing I can think of that you would need to request of the resort would be a crib. If they don't have laundry facilities, take at least 3 more outfits than you think you will need, and double up on beach clothing for the little one
Also, it's been a few years for me, but when I last checked into it, it was really expensive to rent a carseat (it would be cheaper to buy a moderately priced one for a week some places), BUT the carseat doesn't count as luggage so I always take mine along so I don't have to worry about that.
Have a super fun time!!! I'm so excited for you!