Ohio,

you definitely can invite them for October even if you are graduating in December. They can say that they are going to be in the US for a couple of months to spend with their son including attending to convocation ceremony.

I heard that if you received a multiple entry visitor visa for 5 years, you have to come to the US within 6 months of getting the visa. Then you can come and go whenever you want wtihin 5 yrs period. I have not validated this 6 months requirement though. I know one of my relatives came after the graduation on a visa that was obtained to visit the US for son's graduation. They came within the 6 months after getting a visa; however, after the son's graduation ceremony.

I hope this helps.