Your son is old enough to enjoy a sandbox.
I just made a cedar plank sandbox(4' x 4' x 6")for our 26 month old granddaughter. She enjoys it and although not as big and deep as some, it is good enough for two toddlers.
IMPORTANT: teach sandbox safety and ettiquette right from the start: NO throwing sand; how to brush sand 'down' off one's hands and clothes; keep sand in the box and leave the surrounding mulch out. I keep a hose and towel nearby to rinse & dry when done.
Last year, between her 1st and 2nd birthday we used a large plastic storage container similar to the one mentioned in a previous reply. She loved the feel of the sand between her toes. She enjoys a plastic set of little animal shaped molds, a scoop, rake, and pail. She also likes me to 'write' in the sand - another good idea for teaching letters!
Be sure to buy playsand, not garden sand, and keep it covered: I framed a section of wood lattice and attached a scratch resistant screen to one side to use as a cover against leaves and animals, and it looks attractive. Also, when I made the bench seats for two sides I attached each of them with brass hinges so I can easily lift them back to check for insects or spiders who like dark places to nest.
To keep fine sand from flying about in the wind and getting into eyes keep it slightly moist by adding a little water and stirring up the whole batch.