This might be a bit disjointed, so apologies if I am all over the place. (Also, the usual preamble that this is all the opinion of some random on the internet, and not actual medical advice, so, as always, take everything with a grain of salt, and seek actual real life advice, because I could just be some pimply 14 yr old in my parent's basement etc).
Firstly, give yourself some time to process all of this.
This is a huge amount of information to take in. Things like "low IQ", "special needs", "intellectual impairment" are incredibly hard things to hear as a parent. Give yourself permission to grieve. Be upset. And please don't feel you need to rush into action mode. Often when parents get this kind of information, the first thing you want to do is "fix it" - get him in the best school, book in with every doctor in town, put in place all sorts of programs and strategies. It can wait. He is the same boy today as he was yesterday. Yes, things will need to happen, but they don't need to happen TODAY. Give yourself time.
But also remember this. There are worse things than having a child diagnosed with a learning disability. That is having a child who isn't diagnosed (but still has the problem). As hard as it is to hear, the fact that the school has picked up on all of this is absolutely fantastic. Because it means that your son, and your family, can get the support and assistance you need.
Definitely follow up with a paediatrician. If you can, get a referral from your GP for a developmental paed
. (If you post in the special needs section and give your location, I am sure some of the lovely EB ladies might be able to PM you some recommendations). You mentioned that your son has been assessed by an outside clinical psychologist? Who diagnosed him with ADHD, and said he had a "low IQ".
You mentioned memory problems? Depending on the background of the psychologist, and what tests they did, you might want to consider getting some follow-up testing done specifically looking at memory function. Sometimes more specific memory deficits can be misdiagnosed and just lumped into ADHD(inattentive type). Additional testing might help to clarify what his memory issues are. The testing I am thinking of would be done by a paediatric neuropsychologist. They are few and far between, and can be horrendously expensive. I would discuss this with the developmental paed, to see if they think it is worth following up. It sounds like there is a complex mix of things happening for your son (with OT, speech, psychs etc all involved) and they will all be looking at this from their own perspective. A good developmental paed should be able to act like a conductor at an ochrestra, and help bring this all together for you (because I am sure at the moment, it must just all feel like a whole lot of noise!).
If you haven't already, I would start a records folder for your son, with copies of all his reports and assessments. I would make copies of these, so that you can hand them out to any specialists you see while still having a copy for yourself.
On the topic of medication, this is such a personal choice, and I can completely sympathise with your reluctance to medicate. One thing I would say is this. You said
we dont REALLY want to medicate as what we are scared about is that it will change his beautiful personality
but then you also said
I can see it in the last term - he is starting to become more withdrawn and very very tired.
I guess what I am saying is that while you might be afraid that medicating your son will stop him from being who he is, it is also worth considering that medicating him might have the opposite effect, and actually allow his true self to shine through.
It's the same with the school thing. I'd look into the other school. As well as having a dedicated class for children with higher needs, you might also find that as a result, the staff as a whole have more training/understanding in teaching/interacting with these kids, and might be more familiar with specific strategies. Asking the question and having a look doesn't commit you to doing anything, so, when you are ready, I would look into it further.
Again, I will just come back to finishing this massive novel of a post (sorry) by saying that the most important thing you can do for your son RIGHT NOW, is to give him a great big hug, and to take a deep breath. Good luck, and my thoughts are with you and your family.