Can you relate the triangle drawing to something he already knows? For instance, to make a triangle, start like you are making a capital A, then connect the two bottom parts with a straight line.
A good intro to scissors is just to get the hang of opening and closing them by snipping pieces off strips of paper. Let your DS "shred" your junkmail for you
For more control, you can start by drawing a thick line with a marker on a piece of paper and having him cut on the line. Then try zigzags and curves.
Cutting is actually a series of movements. Open scissors, position paper where you want to start cutting, make sure scissors are pointed in the direction you want to cut and the blades are vertical, close scissors partway, reposition (turn) paper if necessary, repeat. It's actually pretty complicated and a lot of kindergarteners have trouble with it.
