It sounds like your son did a great job and obtained quantitative results. He had a reasonable hypothesis and he did a controlled experiment, however, he did not confirm his hypothesis. There is nothing wrong with this and it is very common with science projects and with experiments done in research labs.
Your son should go ahead and prepare his bar graph. The results are empirical; they are actually what happened. Be sure the graph is labeled completely. The graph should be the centeripeice of the display board.
He really can’t say that animals have more DNA than plants because of the results of the strawberries. What about all of the plants and animals he did not test? The best thing to do is to do some additional reading and try to do a thoughtful analysis that will explain the results. Your son should start his conclusion section by saying that his hypothesis was not correct, that animals do not have more DNA compared to plants. He can then distinguish his project with his discussion section. Here are some ideas:
Here is a website that lists the number of chromosomes for various species. Please note that the list contains a mixture of plants and animals, and there does not seem to be any trend in the numbers. Rats and ferns are at the top of the list and rye and mosquitoes are close to the bottom of the list. http://en.wikipedia.org/wiki/List_of_or ... some_count
Why don’t you help your son do a search for the number of chromosomes for each of the species tested? Perhaps there is a correlation between his results and the number of chromosomes?
Did your son do duplicate results on his samples? If so, were the results reproducible? If he did not run duplicate samples, then it’s possible that the normal variation in results from this experiment caused the variation. So the cause would be normal experimental error.
If the results are still not explainable, ask you son if he can think of anything else. Your son can also include a suggestion for another experiment that he might do in the future that would explain his results.