I wouldn't think twice about trying to get him up to speed on Science Fair specifically - there is so much material available on Science Fair projects available at libraries as well as online, that when the time comes, he's going to have plenty of resources available to learn about how to write up his experiment, what expectations are etc. If you're really really wanting to do a write-up etc this summer, you can find good resources simply by googling "Science Fair + 6th grade" etc.
The only thing I'd do at all for experiments is ask if there are any specific experiments he would benefit from completing over the summer that tie specifically to the 6th grade curriculum that he missed - for example, if electronic circuits were studied, is there a lab that illustrates the concepts learned that would help him reinforce those concepts. Ask if there are labs that teach principles he'll be expected to be familiar with. And if you get nothing specific from the school, don't worry about it - if he reads about something he's interested in, have him think up his own experiment
polarbear
