I agree that drawing and handwriting are two different things, and I've seen great artists that cannot automatically recall and form letters. But writing is also more than just the handwriting, and a child with ELD may have trouble organizing his or her thoughts, recalling specific language and writing it down, just as they may have similar difficulties speaking the thoughts aloud. Some people with ELD actually find it easier to write - because if given ample time, they are able to take the time to organize, find the words and get them down on paper. For others, the act of writing is even more torturous. My son for example has trouble with the grapho-motor aspect, spelling as well as organization, retrieval and precision, so he prefers speaking to writing - even with the time constraints.
The issues you describe with word retrieval (explaining rather than naming ex. that temperature taking thing for thermometer, imprecise language ex thingy, stuff, and frustration) seem to indicate possible difficulties. If he continues to feel frustrated and if it impacts his participation and learning, I do think you would be wise to evaluate. Best of luck!
