How did he get the original diagnosis? When getting my sons diagnosis I brought him to a pediatric psychiatrist. The school district also has their own testing. I would talk to the doctor who gave the original diagnosis and ask for further testing and observation.
The odds are pretty high against two brothers both having ADHD. Often the younger child takes his cues for behavior from the older one. You don't metion how old your sons are and whether they were diagnosed by a specialist.
Allmymarbles is correct when she says the younger son, "takes his cues for behavior from the older one." But ADHD is hereditary! So both could easily have it. Age is important. Schools are probably not the best source of testing for autism - simply because it is usually diagnosed before the child is in school - thus the schools don't tend to have enough experience. A pediatric psychiatrist is probably your best bet.
There are specialized places that test for autism. You might try calling the closest children's hospital in your area. They probably have some resources, also ask your pediatrician and he/she might know. Yes, I don't think schools are good at doing diagnostics of any kind.
also, how old are the chldren. I asked my child psychiatrist about ODD and he said it could not be diagnosed officially until age 12.
I mentioned the school district if getting medical treatment was an issue. Sometimes getting specialist with little or no insurance is a problem. I know from friends who are now going through it that there are a lot of early detection of disabilities in the public preschools. I also found out when my insurance and GP gave me a hard time that the pediatric psychiatrist's cash fee was the same as my co-pay with the insurance. Just putting the info out there in case insurance or $$ is a problem. With our ADHD & Aspergers diagnosis out psychiatrist likes to see my son at least every two months.