You may want to see the differences of ADHD and mild autism. Subsequently, you watch for the symptoms at home and gather other feedback from teacher. Then, you decide to meet with specialist at KKH to do assessment and then review the results. Yes, I agree with your husband that all kids are different and progress at different pace. However, on the other hand as an educator they learn to watch for signs of each child, feedback or advise parents accordingly. Because they see our kids most of the time during school hours they know it well. They may be right, they may be wrong. But, nothing wrong to get review from specialist. Personal experience from a friend, she doesn't know the son has mild autism till she go for review and seek specialist assessment and subsequently diagnosed with mild autism.
So, sometimes we shouldn't ignore advise from teachers otherwise it will be too late to recognise and go through the right channel for assessment/review/school.
Hope this helps.