If it's just a aptitude test alone, then he would sure to pass. Many Autistic people are able to pass many aptitude tests, or even pass them with flying colors. However, Autism, and even Asperger's (A milder form of autism which tends to increase aptitude but decrease social skills) would be disadvantegous to a soldier. Soldiers are meant to work in groups. Autism and Asperger's people are usually loners and have a hard time socially functioning, so an Autistic person, unless they work VERY hard, will find it difficult to be in the armed forces.
Those "Army of One" ads would be enticing to such folks.