As per my experience ~ Don't worry, your son is still young to control a pen. For my son, he cannot hold the pen well before 3 years old. But now he can draw and paint quite good. He can write A-Z in Cap and Small letters very well and now he also try to write some simple Chinese Word. Before he 油顏色出晒界 and only like to use one or two colours, like blue or black only. But now he can paint ok and use difference colours. And he can draw some pictures, like girl / boy, dog, butterfly, etc ...
Therefore, don't worry and he will improve a lot with a short period