Has my son got ADHD?

We have had our youngest son (8 years old) examined by an ADHD specialist who said that he shows a lot of the ADHD characteristics and that he is very intelligent but is just doing enough at school to get by. However, his current teacher states that he doesn't have ADHD because he is fulfilling all the average criteria for his age, despite the fact that in our opinion his writing and spelling has deteriorated during the last year. His previous teachers have said that they thought he had ADHD and it was on the advice of one of them that we sought help. Out of school he is hyper-active all the time, If we ask him to do anything, we have to ask at least 3 times and have to make sure we have eye contact, he is impulsive and he is always jumping around. If he is asked to go and fetch anything, he often forgets what it is and will start playing with or on something and not return. But without his current teachers agreement we cannot give him the help that he might need. What should we do?