I would like to help you with your question.
Bedtime during the summer does seem to change in most households. In part, it so kids have a greater chance to play outside with friends, to relax, and to enjoy life after a long school year.
In my experience, a 9 1/2 year old can get plenty of good rest if they are in bed between 8:30 - 9:30 p.m. Given that your son gets up at 7:30 a.m., he is getting 10 hours of sleep or more depending on how fast he falls asleep and if he wakes at all during the night. This is a very healthy amount of sleep. In fact, I think you are doing a fabulous job if you have been keeping to this schedule!
On the weekends, there is no reason he can't stay up later...particularly if there is no important activity that he must attend to early in the morning. Certainly, a little sleepiness here and there from having stayed up a bit past his weekday bedtime is not going to harm him in the least.
In part, I would also look at what is going on at night that he wants to stay up for...or that might be particularly enjoyable for him....
There are likely to be any number of weekend activities like camping, parties, weddings...all kinds of things that would be fun for him that you would want him to participate in. So...again...a little flexibility on bedtime would be good.
For a growing lad, 10 hours of sleep is very healthy.
You are doing an excellent job of setting a consistent bedtime routine. Kids really need such consistency. The summer, as I've said, can be just a bit less restrictive.
I hope this has answered your question for you and your son. If you would like to chat further, just let me know.
I appreciate the opportunity to assist you!