Yes, after 5/6 years old is better. The result will be more reliable. And if you have already let him do IQ test at such a young age,it will affects the result of the second time of IQ score yrs later if you want him to do the test again yrs later.
Well, I don't think it's necessary for you to bring your son to IQ test. The CP will only tell you the facts that you know already since you're your son's parent and you know him well. I don't think there's any advantage to hold a IQ reports at your son's age.
The focus should be on the problem your son encountered at school. I don't think IQ test could help. In fact, the situation is far different due to the teachers' experience and the style of the kinder. If the way of teaching is very traditional and the teachers are not fixible,the situation will be more serious. Some will be OK when they go to primary school. Take it easy!