Can2K, the motor skills required to draw are different from the motor skills required for handwriting. It's not uncommon to see this in kids with dysgraphia - the ability to draw (even really well) without issues, but struggling with handwriting. When you use handwriting, you have to execute a series of pencil moves in a specific order/fashion, and you lift your pencil up off the paper every time you write a letter. Cursive is often easier for kids who have handwriting challenges because the letters are connected which means you don't have to pick the pencil up as often. For some kids (with dysgraphia) there is a challenge with remembering *how* to draw the letters - it isn't automatic like it is for most people. When they are drawing pictures, though, the need for rote automaticity isn't as important.
The white board might also be easier for any number of reasons - it's a solid surface so a kid who has a poor grip or who exerts too much pressure when gripping might find it easier to write on. The markers are most likely larger around than a pencil is, so they might be easier to control or grip. There aren't any lines so he doesn't have to concentrate on making letters "fit" within lines... etc...
It is possible his motor skills are delayed, and it's possible there might be more of a challenge than just a delay. If you have a comprehensive eval to get more info about whether or not your ds has DCD and what the impact is, you'll learn more about his handwriting ability/speed/challenges if they exist.
Best wishes,
polarbear
