seriously, I don't think its a racist problem here. Like you say, it was ok, when he was in preschool and he has 'brown skin' friends too....
At the age of 7, kids doesn't really tell the whole truth, not that they choose to lie, they just keep half the truth....for fear of being scolded, for fear of being disapprove, etc
Remember his first reply was the 'bumping' incident, there may be more than just the 'bumping', it may be a bulit-up of events...
Don't worry, keep an update of the situation by initiate to ask, so is Ebony disturbing you today? Let him know that you are concern, but doubting his honesty. Whatever reply he gives you, just accept it, don't rebuke him.
If you really think its racist problem...can try this..."son, how would you feel, if one day no one wants to talk to you or play with you anymore?....their reason is because of your skin colour, not because you are a naughty boy...do you like that? do you think its fair?"
Let the child think...he will have the answer.
http://allaboutyourchild.com