Something not right with my 7 yr old but no one knows what.

Hi, I am new here and looking for advice about my 7 year old son (7 in June). 

I have felt for a long time that something isn't quite right, he was a very difficult baby, cried lots, hard to settle, difficult withe feeding and slept very little. He is our second (and last) child and our first born was quite a settled baby so kept thinking that we had been spoiled with her and that it was just different second time round. 

 As he grew he never seemed to quite meet his milestones at the appropriate age. He was late to walk, talk, potty train etc but always got there eventually. He continued to be difficult, very unsettled and as he got to 18 months he had extreme tantrums which seemed more severe and more frequent than other children of the same age. Again we thought he was just strong willed etc and would grow out of it. However they continued.  When he started nursery we noticed that his interests were unusual, becoming obsessive over batteries, soap and recycling items but again't put it down to just him and thought he'd grow out of it. 

All of this continued. He's 7 now and still has frequent tantrums or meltdowns in various situations, especially in shops, theme parks and other days out, when visiting, especially when there are lots of people in the house. Even at home he is often unsettled, he is aggressive towards his sister verbally ando physically and often shouts at us and other family members too. He often refuses to do as he is asked, or will do things his way. If I ask him to get dressed he will (often after delaying it) but will put his top on backwards, or will refuse to wear socks, it's like everything has to be on his terms. He will happily play with other children (although this is quite a recent thing) but again only if he's in charge of the game, and if they refuse he becomes aggressive, or if he's at school when this happens he becomes upset that they are no longer his friend. 

 At school he is described as having perfect behaviour and school have no concerns other than he isn't progressing well.  They tell us that he's doing okay ( though at one point we were told his progress had stopped all together) but according to his report, he is well below average in everything other than his reading, even for effort he is below average. He hates going to school, he complains every morning that he hates it yet when he's there he's fine. 

 At home he just seems full of something, I know that sounds strange but it's the only way I can describe him. Like he's full of energy or anxiety I can't work out which.  He tells me he's excited, the other day he was crying because he wanted a sleepover at Nans house. He was adamant he wanted to stay on Monday night. So we arranged it and on Sunday told him he could sleep over the following night just as he'd wanted. He went into complete meltdown saying that he wanted a sleepover, I explained that he was having one and he replied I want one tonight. I left him for a while then went back and asked why he was crying when he was having exactly what he wanted and he said I don't know why I just feel excited inside. 

 He's under the educational psychologist for 18 months who has just moved the referral onto the CCAT asking them to assess him too. But he keeps saying that he's only done this because things haven't improved, he doesn't really think there's cause for concern and he doesn't really fit any type of disorder such as asd or ADHD.  

 I'm so sorry I've written an essay!! I just don't know where to turn next. I have a feeling that when he is assessed nothing will be picked up and we'll be back to square one. I'm in turmoil, part of me wants to treat him as though he has asd or something else but when experts are saying there's probably nothing wrong I can't help but see his behaviour as him being awkward and nasty. Deep down I do think something is going on, he is such a lovely little boy, his behaviour is very difficult to manage though.