Welcome, Chaya!
Am I correct that this assessment was done in Spanish? (I notice that you've listed nonstandard subtest names, which I assume resulted from back-translation from English to Spanish to English).
I'm going to start with some context on testing of this kind that is important in interpreting the results. First, it is generally considered inappropriate to interpret the scores as age-equivalents, except in very rare cases, so I've tried to give some rough approximations for what the scores you've reported actually were in normative terms, with the standard subtest names. Since I don't know which version of the WISC-IV was actually given (Spanish or English), or what the child's actual dominant language (not always clear with dual language learners) or cultural context are, I'm not going to convert these to actual numbers:
Block Design: average
Similarities: average
Digit Span: (beginning of) average
Picture Concepts: extremely high
Coding: very low
Vocabulary: average
Letter-Number Sequencing: average
Matrix Reasoning: average
Comprehension: average
Symbol Search: average
Second, interpretively, we have a few additional cautions: this is a fairly old version of the test (how old depends on where you are in the world and whether it is the Spanish version), and may or may not be based on norms appropriate to the cultural/linguistic/educational context of this child, so the results on any given item or task have an elevated risk of being imprecise in either direction (too high or too low).
So when we look at the results in terms of scaled score classifications instead of age-equivalents (which, again, there are many excellent psychometric reasons for avoiding as much as possible--but that's another, much longer story), we see that most scores are comfortably in the average range, with only two notably outside of it: Picture Concepts, which is indeed in the extremely high range, and coding, which falls in the very low range.
But let's say these do tell us something about his cognitive profile (and that is definitely making some big assumptions). It would suggest that, in the context of generally age-appropriate thinking skills, he is much better at concept formation using concrete images than he is at abstract verbal concept formation, and that his complex pencil skills are slow (notice that he did fine on Symbol Search, which is timed exactly as Coding is, so it's not speed itself that is the issue, but how it interacted with the specific Coding task--which could be due to anxiety, as you suggest, or possibly due to fine-motor coordination and handwriting factors). For what it's worth, the exercise is intended to be--as you note--very easy. The point is completing it efficiently, which can be affected by many different factors, including hand skills, anxiety, attention, fatigue, visual tracking, etc., in addition to pure motor speed.
I would not say the data support a poor short-term memory, but it is possible that he just compensates very well using other methods. I will note that it is quite unusual for exceptional musicians to have low working memory skills, though, as there is some association between rhythmic accuracy/precision and long short-term memory spans.
While it can be true that those on the autism spectrum test less consistently early in life, I would be cautious about interpreting his testing results with a view to diagnostic classifications such as those you list, based only on a few numbers. All of the diagnoses you mention are typically made using multiple measures and a high degree of clinical training and expertise. Asking the question is certainly reasonable, especially if you are seeing other signs in daily life that suggest this, but be open to a very different answer than the one you expected.
Finally, I am happy this young person has so many adults in his life who care about him, and who are trying to help him grow in every way! A child cannot be loved by too many people.
