If he seems happy, don't worry.
They like sand and water play at this age but tend to be very messy so the shower curtain is a good idea if you do this indoors. Pouring water or dry sand from one container into another, can entertain for a good while. Digging and using little toy vehicles in sand can also provide nice quiet fun.
A paintbrush and basin of water outside on a patch of concrete can provide fun 'painting', and chalk is also nice for making marks outdoors. If you can get someone to paint a section of interior wall with blackboard paint, maybe he could use chalk indoors too? Writing on a vertical surface is good for developing pre-writing skills, which is a bonus.
The DCs all liked to use my salad spinner with wooden blocks in it but maybe they were bit older than your DS when they could spin it. They loved emptying my pot and pan cupboard and pretending to cook with my wooden spoons, under the kitchen table, using the wooden blocks for ingredients.
We did a good deal of reading together too, nice if you're not up to chasing someone in the park or if the weather is a bit inclement. And where would I have been without the TV?
Playing 'with' other children doesn't happen until much later. They like to play alongside others but conflicts can arise over toys so supervision is needed, which can be a pita.