CarCar, you're getting a lot of people saying to keep looking, you probably haven't got all the answers yet and without them, you may be seeing it from an incorrect point of view. The kid he pushed to get him to stay away - your son sounds impulsive and frustrated. Why did he want that kid to stay away? Is it possible that your son was being pestered by this kid in some way? Because by crikey, it happens a lot.
But your main question is - what do you tell the other parents when they ring up to nag you about your son?
I agree with you about not telling them he has ODD. Frankly, that is none of their business. However, you're feeling defensive and put upon with them (understandable - our kids make us feel so embarrassed sometimes) and you feel pressured to give them some sort of explanation. So try this on for size, for starters -
"We're in the process of getting some sort of understanding of what is going on with him. We still haven't got definitive answers but we're doing the best we can."
In the meantime, keep records of what he's accused of doing, and any other information you can glean about these incidents. You say he has friends - ask them what they think happened. But ask them when they're not with anyone else, so they can't get stories straight. You want the truth, not a doctored story. Then write it down. It mightn't make sense now but further down the track it will and you will be glad you took notes.
You say the problems have been particularly bad over the last few months - can you think of any possible connection? A change in teacher? New kids at the school? Kids leaving? Anything else?
But people who ring you - you're not obliged to divulge anything. Give them an answer that tells them you know about it, you're taking steps to identify the problem in detail and you've already ruled out a lot of possibilities. You are caring parents who are good people, trying to do your best. You can't do more than your best. You welcome information but can't tell them anything that you don't yet know yourself, because this is a process that takes time.
Good luck. And do get a second opinion, from someone with a relevant medical degree. Maybe check out http://www.childbrain.com
and see how he scores on their informal Pervasive Developmental Disorder (PDD) questionnaire. If he scores anything relevant, print it out and take it to a doctor of your choice. It might speed things up. Even if he scores positive for Pervasive Developmental Disorder (PDD) and later on someone says it's not, the sheet can still point to areas where you're concerned, and help a doctor get a feel for the problem.