I wouldn’t push trying to get him to draw. He probably doesn’t have the eye-hand coordination he needs to produce a product that would please him, so he resists doing anything at all on paper. He is too young for coloring books. They will simply frustrate him, so don’t buy any more for at least a year.
Get him some finger paints and provide a place for him to paint, preferably a fairly large surface. Or buy a tablet of easel paper, some washable paint, and a brush or two and let him fill the “canvas” with color. (You don’t have to buy an easel. It will work just as well if you put the tablet on the floor on top of some old newspapers, or prop it on a bench in the garage.) Also, get him some modeling clay and encourage him to work with it. Join him if he wants you to.
Whatever you do, resist the temptation to ask, “What is that?” when he completes a painting or a sculpture. If he spontaneously says something about it, ask him if he wants you to write that in the corner so both of you can remember it. That sort of thing will help him get used to the idea of print as containing a message and will encourage his interest in those strange squiggles. And, by all means, prominently display his masterpiece someplace in the house.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.