Hi joanne. Sorry to hear this- you must be worried about your boy. I hope you will find that there's lots of help around for him though.
As Davros says, you can indeed ask for your son to be assessed yourself. There's a proforma letter by IPSEA here. I'd agree that IPSEA is generally v good on statutory assessment/statements and you'll probably be able to find all you need to know on there if you decide to go that route.
However, personally I would definitely recommend speaking to the school first and, if at all possible, getting their support/advice before deciding whether to ask for your son to be assessed. Does the school think he needs a statement? Why or why not? You really, really, really need to know what the school thinks because the LEA will consult with them to decide whether to assess your son for a possible statement. If the school reports no worries or only mild ones, the LEA will almost certainly refuse to assess. And if you want to challenge a decision not to assess you will need lots of good evidence about your son's needs and, ideally, a lawyer to present it to tribunal- not always a stress free journey, IME.
So I'd suggest that you save that approach for when you're sure it's needed and try to work closely with the school in the first instance- this will be important in any case, obviously, so that your son gets the best support. Have you spoken in depth to his class teacher and asked how he is doing and what extra help he is getting? I'd agree that requesting a more formal meeting with the class teacher and SENCo if the teacher alone can't give you the answers you need can be helpful. Is your son on the School action or School action plus stages of the school's special needs register? What support does that give him? Is it enough? If not, would the school agree that a statement is necessary?
Really HTH, and best of luck, joanne.