Can an adult be diagnosed with autism, or is it something he has to have from childhood?

Yes. : Diagnosis includes looking at the person’s medical history, watching the person’s behavior, & giving the person some psychological tests. But, it can be more challenging to DX an adult because it is not always possible to know about the person’s development during the first few yrs of life, and a long history of other diagnoses may complicate an ASD dx. http://www.cdc.gov/ncbddd/autism/topics.htm.