As for being dxed with both, it depends on where you live and which diagnostician you happen to see. If you live in the US, the DSM specifically states that an ASD and an ADHD cannot be diagnosed together. However, as that stipulation will likely change with future editions of the DSM and seeing as how many auties prove the accuracy of that criterion false, they can certainly occur together. "Occurrence" and "diagnosis" are often two different things.
In the UK, Australia, and elsewhere in Europe, there is less strict adherence to that ICD stipulation and will more often see comorbid dxes of an ASD and an ADHD. Although there will still be diagnosticians who refused to dx both.
However, in real life, you will rarely find an autistic person who doesn't show at least some of the behavioral signs of ADHD-- some even fulfulling the full additional criteria for an ADHD.
If you are living in the US, you may find it difficult, if your son does actually fit BOTH an ASD and ADHD, to get him both diagnoses. However, if it comes to a matter of one or the other, getting your son the ASD diagnosis will likely be more beneficial for him as far as providing for accomodations in his school.