Apart from the social skills, he sounds a lot like my DS at 5 or 6 and he isn't dyslexic at all, he has DCD, the main symptom of which is dyspraxia. I don't think he learnt to read phonetically at all, he learnt by memorising the words but like your DS he has a great memory and once we rote learnt the key words it seemed to be enough and he was away - he really really struggled the first 1.5 years of school though.
Even with reading sussed, struggled with writing -he couldn't write his name and surname as we saddled the poor chap with a very long first and surname and his Yr 2 teacher said he didn't have to bother trying to write his surname as it just took so long! He is 12 now and has a reading and spelling age or somebody 4 years older so despite not learning to read by the usual route he has managed perfectly well. However, his written work is very scruffy and difficult to read, he cannot lay anything out properly and he makes the most stupid grammatical errors which he knows are wrong if you were to ask him the rules but he just can't see in his work - things like putting capital letters randomly throughout his work and never using a full stop or comma.
That isn't to say that your DS doesn't have dyslexia, but he may not. My boys' infants school only screeens for dyslexia, it doesn't do the tests there because it isn't ususal for children of that age not to be able to read for many other reasons. Hopefully, given that your DS is obviously a bright boy that won't be an issue when he sees the EP and they will feel able to diagnose now.
If you suspect dyspraxia the EP won't be able to help you. They aren't qualified to give a diagnosis for that, you really need to see an occupational therapist which means a referral from your GP. If you get a diagnosis of DCD (I was told dyspraxia is a symptom not a diagnosis by the occupational therapist
- nobody has ever heard of DCD so saying dyspraxia is easier) the OT can give you some exercises to do and hopefully the school will do them with your DS as often as they need to be done - daily for my DS. The OT trained one of the LSA's in what exactly had to be done and OT came a couple of times a year to see how DS was getting on and update the programme. They even came in early with DS to do them before school so he didn't feel left out.
As for what to tell your DS, if he knows he is struggling in some ways, you could just say what I did and tell him that you are going to see somebody who might be able to make things a bit easier but first they have to find out what he can and can't do. My DS accepted this with no worries. I didn't go into the ins and outs of what might be wrong, just that he might need some extra help and this is the way to get it.
Good luck with your assessment though. I hope you get some answers sooner rather than later. It is all a bit of a worry isn't it? You have my sympathy. 