Boys and girls develop different skills at different rates. Girls usually develop small motor skills (coloring, playdough, etc) earlier and boys tend to focus on large motor skills first (running, climbing, etc). So, even though he is about the same age as your younger daughter, his development will be very different.
Let him be content with poking holes in the playdough or scraping the colors off the crayons (maybe get him his own big chunky set so he's not ruining the girls' crayons). Let him paint his "mess" - with aprons and lots of newspaper to contain it! - and just remember it's more about the process than the product. His expiramentation is helping him understand the world around him.
Also get some active toys - a soft ball he can throw, and maybe a nerf soccer ball or something for kicking. He can kick it up and down the hall, or you can all bundle up and go outside. Kid slides and other play sets are good for small motor skills too. One year, we bought a pop-up play house (made of cloth and the wires that fold down small and easy) and it gave our kids hours of entertainment - crawling through the tunnels, poking up out of the top, etc.
Also consider outings to a nearby park or other places. Even a walk around the block (in the stroller for the little ones, if you can) is a change of scene, stimulates their brains, and is good exercise. Your girls may be less interested in the large motor skills right now, but they need chances to work on those as well as the litle boy does.
Do set limits. Balls are for throwing, but not other toys. We don't kick the ball in the kitchen. Intercept and prevent behaviors as much as possible, and redirect to something more appropriate.