Hello L.-
I have to agree with Comer and Christina with regards to a spanking. This is a DANGEROUS thing and you really can't reason with a 2 1/2 year old. We have to choose our battles with our kids and decide what needs serious correction, and this is one of those situations. It is a matter of safety.
I personally like Dr. Ray Gaurendi's advice
(http://www.drray.com/) . There are the 4 D's that need serious correction - disobedience, danger, disrespect, destruction (I think I got them all correct).
One thing that I have always done with my children, is when we get out of the van, they are to all put their hand on the side of the van. This keeps them in one spot until I am ready to move. Currently my 29 month old does it, and listens very well. There are times when she takes her hand off, but I quickly put it back and tell her to keep it there.
In all these things, I PRAISE all of them (LOTS) when they listen and will sometimes give them some sort of reward as soon as possible. The word discipline means "to teach" and that is our responsiblity as parents. And as Dr. Ray says (respectively) it can take you "a million times a day" repeating yourself for them to even start to get it.
If you are consistent, follow through, show your son that this is serious, he will eventually learn. Children need to hear us use the word "obedience" and that they need to obey their parents, and we shouldn't have to always give them a reason why.
Have faith in your abilities as a parent. As the title of one of Dr. Ray's book reads, "you are a better parent than you think".
Good Luck
C.