Hi Kelly
Now I am not an expert on this subject, but I just wanted to try and give some response as you hadn't had one yet. As I understand it, the main purpose of the medicals seems to be to evaluate how much (if any) help a person may potentially require in regards to their medical condition, and how much this might cost the country/government.
I think it may depend if your child needs. e.g. if he has any regular physio, speech therapy or medication. I understand that CP can be hughly variable, with some having such a mild form that it barely inhibits their day to day life at all, where as others are much more impaired by it, and need a lot of essential personal care. It really may depend where your so falls within this.
I know my answer isn't really very helpful, I hope someone who has maybe gone through this may post later and be able to tell you much more than I can. I have seen that a lot of people, with many varying conditions have still been able to emigrate. Your sons CP may not mean the end of that dream. You only need to take a look at the CP atheletes taking part in the Paralympics right now to see how this condition doesn't have to stop people being whoever they want to be!
Good luck.