Is your boy losing interest in everything? Then you need to talk to the doctor. Is he losing interest in drawing because he has gotten interested in something else? That's OK. There are lots of interesting things in the world and he's discovering more of them. You want to encourage this.
(If he were my son, I'd let him use crayons because color is an interesting thing in itself, apart from line. I'm not talking about coloring books.)
Don't panic, and don't push him unduly on the drawing business. You don't want him to start to think art is some sort of punishment. Are you an artist? Go ahead and do your own work. Let him see pictures, but don't do it to pressure him. In time, he might well come back to this interest again. Two years old is still very young.
I remember (vaguely) a story about the famous American artist N.C. Wyeth, who had several children. Nearly all showed an ability in art or music, and he and his wife gladly encouraged them to go in those directions. One of the children was different (of course, it was the one named after the famous father). This child lost his interest in pencils and developed an interest in making wheels turn. Far from being disappointed, Wyeth said, "This one isn't an artist - he's probably going to be an engineer!" So they encouraged him to follow the bent he already had. And sure enough, Nathaniel Wyeth became an award-winning inventor and mechanical engineer.