Sorry, I see he was diagnosed in 2006. Still his re-evaluation was 6 months ago and he still fitted the criteria then. From you and your son's point of view you need some constancy! If he is really borderline, in six months time he might fit the criteria again! Even if he is borderline, because of the lifelong difficulties/differences I would probably push to try to keep the diagnosis because of the supports/services it brings with it. Or does he now fit a diagnosis of PDD NOS? Again I would recommend speaking to the National Autistic Society because they will have a much greater understanding of the different diagnosis and the impact on services that each diagnosis brings.
It is just so ironic that so many people post on the forums fearing a diagnosis, when those of us who have one fear it being taken away!
I am a parent, not an expert. I would recommend you contact the National Autistic Society in your country to get some advice.
Is getting a private assessment an option?
As you already have a son with autism you will already know about the DSM IV diagnostic criteria (if not go onto the autism forum, click on the Health Page icon on the top right hand side and go into Behavioural Characteristics behind a Diagnosis of Autism). Does he have difficulties in the three areas ie. difficulties with speech/language, social interaction, lack of imagination, need for routines/rituals or repetitive behaviour, sensory issues?
Why did school ask for a re-evaluation. I presume that they saw him demonstrating some behaviour that they thought made it likely he wasn't on the spectrum. But having said that, many higher functioning autistic children can demonstrate 'typical' behaviours, but they may not be able to do it all the time, or may have problems generalising that behaviour into another similar environment.
He has only been diagnosed 6 months ago, re-evaluations usually take place once a year. They are usually a multi disciplinary team evaluation with Speech and Language Therapist, Educational Psychologist, Paediatrician, possible Occupational Therapy as well. This is because, as you know, children can be variable day to day and throughout the day. All these experts will carry out assessments and observations. Has your child been diagnosed through education or health? If it was through education I would go to your GP and ask for a multi disciplinary team evaluation through Health.
I would probably argue that, as you've said, he has only been diagnosed 6 months ago and that they should look more carefully over a longer period of time before withdrawing the diagnosis and the support that provides. And I would start a daily diary at home writing down examples of behaviour considered to be on the spectrum.
Having said that, it would be wonderful if he was just copying his brother, and if that is the case then it should become more obvious when he socialises with other children at school that he is fitting in with them, but from your point of view you will need to see this for yourself.
From your post your child is now 4 years old?