Hi A.,
Do you have a school directory that you can use to find the names/addresses/phone numbers of neighbors whose children attend the school? If not, you might find out if someone in the school office can help you find a parent to help; maybe you could take the kids to school and the other parent could pick up after school.
Having been a latchkey kid when I was about that age, I would highly recommend finding a home or other place with adult supervision rather than having your stepson home alone for three hours or more after school each day. The PACE program another mom mentioned sounds great; here is a link:
http://www.risd.org/schools/rte/PACE/pace.htm
It could be well worth the investment in your stepson. I don't know the situation that brought him to live with you and his father, but I hope he doesn't feel like he's a financial burden. Presumably his father does not have to pay child support now that he's in your home, which was likely much more expensive than an afterschool program.
Being a step (parent or child) can be hard. Blessings to you and your family!