My son was diagnosed just recently and the behavioural pediatrician said he had a very low threshold for diagnosing children with autism. (He is 4 1/2 and is also mild) I had lots of tests; two hearing tests, blood tests, saw a geneticist, neurologist, speech, OT, etc for about 18 months before he was diagnosed. I then saw this Behvioural Dr and he diagnosed him within 40 minutes. Even this felt very quick for me and hard to come to terms with. It is a big label to put on a child even though it does not change who they are.
With this child, I know he is a family memeber but do u see him very often? Have u spoken to his parents about his behaviur at home and is it different to what u see? My son is excellent in an environment where there is routine like school, but loses it often at home, public places ,etc.
Do the tantrums u descibe have to do with not getting something he is wanting or obsessing about? My son is just the same and often unmanageable. I am just saying what u see might not be the whole picture..
I still have not told half of my family because i know they would openly laugh and not accept it..and i am not willing to add these stresses to my life.
It sounds like u are wanting to know more about it to help him and that is positive.
Good Luck