Assuming this is a grocery store typical rectangular cake with flowers in one of the corners, I would try this...
Take a butter knife, small spatula or spreader and gently scrape the flowers from the top of the cake. If you chill the cake first, the flowers may come off easier.
You may have some flower residue left in the area, so you will need to cover it with something. Depending on your son's age, you could use matchbox cars or any other small toy he is interested in.
For an older son or if you have a small cake, try spreading some extra frosting on top and sprinkle the top with mini M&Ms. This would tie into the probably orange/red trim frosting around the bottom.
I may can give you other ideas if you can give me more info on the cake and your son's age.