One of the most helpful members of the school staff is the Speech/Language clinician.They have diagnostic tests that pinpoint a child's language (reading, writing, speaking, processing) difficulties. If I had a son showing this little signs, I would ask if I could have the Speech/Language clinician/pathologist give him the tests she felt were appropriate. If a clinician has been involved, I'd ask for some other tests.They are well trained in knowing which tools would be best. This IS the beginning of the school year and it's a time when clinicians are most swamped developing Individual Educational Plans for their case loads, so it might take a while. The Speech/Language person at your school might be able to give you a the name of a private clinic that he or she trusts if you want more immediate answers. Sometimes insurance covers the cost. But I'd always defer to the school Speech/Language person.
Dyslexia is an ambiguous label and other dynamics may be going on with your son. If his vocabulary test is high that IS an indication of intelligence especially if its the Wechsler Test. (Speech/Language is not my field and it has been YEARS since I've been in education, but this is what I remember).
I, myself am a very auditory learner. I hear every word in my head when I read. It's called sub-vocalization. I get most of my information from listening. I love radio and am always listening to informational radio in the car or at home. I am a writer, but I do like to create recordings.
I think the most important thing right now is not to panic and to give your son as much positive feedback in his skill areas as possible. I'd ask the teacher if oral reports were an option, but I would want to be sure it was an option for the class at as a whole. You don't want to single him out and make a potential non-issue into an issue. However, you also want to cover all bases. It's like walking a tight rope. You have to be sure you don't go overboard one way or the other.
I remember when my little girl started stuttering at 2. The most important reaction to a little child who has started to stutter is to not respond to the stuttering at all. You don't want to lock a negative in. Same with this.
At home, I'd just continue reading with him, having him read to me. Seeing you write some thank you letters, cards, etc. is a good model for him. Whatever subject he's interested in, that's the subject for him to try writing about.(PS. Is he left-handed?) In that case, I'll get back to you :) I'm left-handed!