Hi and Welcome!!
It sounds like your hands are pretty full. I think if you work to find all of what is going on you will be able to manage things a little easier. Most, if not all, of us came here with ADHD and/or ODD diagnosis. Pretty much, if you handle the underlying problems, the ODD will go away. NOT always, but most of the time.
I think your son needs to be tested for learning disabilities, esp dysgraphia. This is like dyslexia for handwriting, or that is the easy explanation our developmental pediatrician gave us. It way over-simplifies it, but gets a bit of the problem through. It can explain the 50 minuts sentences.
With ANY child where ADHD is suggested, I think seeing a pediatric neurologist for a sleep deprived EEG should happen. Our daughter was diagnosed with inattentive ADHD. I pushed and shoved until we got in to a neurologist and he did the sleep-deprived EEG. She has epilepsy - a form where her brain just cuts out for a very short period. She literally would have 90 or more seizures an HOUR. Very hard to learn when you are missing that much. I would NEVER have caught this without others on this board strongly suggesting this testing.
It also sounds like having an occupational therapist test for sensory integration problems is in order. The school has OTs and will test if you request it (chekc the Special Education forum to see how to formally request testing). My youngest sometimes tantrums, shows signs of ADHD,etc... but treatment for sensory itnegration disorder (Sensory Integration Disorder (SID)) has stopped most of it.
Psychiatry is a difficult field. So much of it is subjective. One way to keep things on track from the parents end is to create a Parent Report. this is a way of keeping records that some moms here developed. I have found it invaluable - I keep all the things related to my difficult child in the 3 ring binder. I have copies of the report in there so I can give them to doctors, therapists, whomever. I am careful in only giving people what they need to treat my child. Someitmes info can be misused, so some care should be taken.
If you go to the FAQ forum, there is a thread (the second one, I think) on Parent Input. I think it would be helpful for you, just to get things organized. I put a small photo of my son on the beginning of each section (and partway through if it is very long). This helps the reader remember WHICH child, and that it is about a PERSON. The photos copied very well when I made copies.
Well, sorry if I was too long. Welcome to our little corner of the web!
Susie