My son started to ride his bike to/from school when he was in 4th grade (9 yo). It was scary, but I think it has made him a much more confident, responsible boy.
I do not beleive there is a legal age for bike riding alone. There are guidelines for being home alone and for supervising a younger child. As far as I know, there are no laws...you just have to stay out of the area of endangering your child and I don't think riding his bike would qualify.
The thing that you need to impress upon your shild is that most of the time he should think and act as if he is invisible to everyone in a vehicle. People have gotten better about watching for children on bicycles, but everyone is always in such a hurry. He needs to watch the cars around him and be especially careful at driveways to retail stores. Look up and go over the local bicycle laws with your son and make sure he knows the hand signals. Most city bike laws prohibit riding on the sidewalk, but my children always ride on the sidewalk except when we are out on family rides...dad in front, leading and me in back, watching and coaching. Ride with him a few times and make sure he is paying attention and being safe. Impress on him the importance of a helmet and that he can get ticketed and you will be fined if he is not wearing it. At this age, it is "not cool" to wear a helmet, so go over this A LOT! I think having him memorize #s, keep track of time and carry a little money are all good ideas. Most Jr. High/Middle School's issue students a picture ID within the first week or so of school. Check with his school. However, if the school doesn't do ID cards or you are worried, you could make an ID of sorts on your computer and laminate it in a luggage self-stick laminator pocket. Another option is paying the DMV $25 for a photo ID, which would be kind of a waste since they change SO much in 6-9th grades.
If you are so worried, get him one of those prepaid cell phones that will only dial out to numbers you program in. Your number, home, dad's number, dad's work, grandparents and/or a neighbor, so that if he gets lost or feels scared he can call. I would also get out a city map and show him all the ways he can get to school/home, but have him use the safest, most direct route. It is scary, but you are really doing him a great service by allowing him to test his wings and become responsible. I woul also have him begin doing chores for you, if he's not already. Being responsible means more than just riding into the sunset on his bike. Email me if you want to chat more. Try not to worry too much and think of the time you will have to do something else now that you don't have to drive him to school. Take Care!