I have never sent DD to preschool so I honest would not know how they do it, but I do not think they expect the kids to sit down for too long at a time either. Most learning is done with crafts and color pages. Children also tend to "copy" their peers so if everyone else is sitting down your DS will not want to be the only one running around.
Here are a few activities that we have done throughout the last 2 years:
We took a basket (I just used the washing basket) and colored balls (you can use anything that you have in a variety of colors). Scatter the balls around the house or yard and go on a hunt for colors. Tell DS to find a specific color and reward him in some way for bringing the right one(we did praise only, but use what works for you). Play this game often and start eliminating the balls or items with the colors that he knows well. After a few days you should be left with only a few colored items and repeat the game untill he know the colors well or is sick of palying it.
We took construction paper and cut out the shapes that we wanted to learn. Then stick them around the house where they are visible. Play some music and tell him when the music stops he has to run to a shape that you shout out. This is great to get rid of some extra energy.
Another things you can do is cut out smaller shapes and tell Ds to match the smaller shapes with the big ones that are scattered around the house.
The obvious thing to do with shapes and colors is to also point them out in everyday life things. While driving tell him that the STOP sign is a hexagon and so on.
We did letteroftheweek.com and starfall.com. We also printed out letters to color. We play with foam letters in the bath and I made letters from sand paper as well. We make words with our wooden blocks with letters on it. Talk about the letter names or sounds at every opportunity that you get.
Count everything that you get an opportunity to with him. DD did not want to learn to count at about 3 and I just continued to count things in front of her. She now counts to 20 and we are working on number recognition right now.
In the morning we go for walks and identify the numbers on the houses. If she does not know it, I tell her the number and we try again tomorrow.
We also play sidewalk chalk numbers (she loves this). I write the numbers 1-10 on our driveway and she gets to do circles around them. Then we call out a number and she has to find it and jump on it. I started with 1-6 then moved on to 10 once she knew those. If I find she has trouble with a number, I ask her what she wants me to draw for her and she always has a suggestion. So whatever she wants me to draw I try to incorporate the number into it. Yesterday we had trouble with the number nine and she wanted me to draw a mulberry bush so we could run around it. I drew one and put a nine in each mulberry. You get the idea...
Please excuse any rambling, bad spelling and use of punctuation. This was typed with both DD's asking for my attention.
Good luck and let me know if you need more suggestions.