It doesn't sound to me like he's going to be home alone much with that busy schedule.  If he is a normal 14 year old he should be fine on his own though.  Depending on what state you are in he may or may not be eligible for work at 14.  If he is then a summer job is great way to build character.  One of the problems we parents have is that we feel a need to micromanage our kid's schedules.  Remember that he's still just a kid and needs some of his own time to hang out with friends.

You may want to check into a lifeguard position too. This would require some training from the Red Cross. Also, I think it is fine to let him stay home. Just make sure he has other activities going on. It may even do him good as it will give him some independence. You can always give it a try and if things don't work out go to plan B.

Wow, what a summer schedule. What would your step-son like to do this summer? Considering this question may help shed some light on your question. When kids reach the teen years they have wonderful ideas about how to spend their summers, some of which may not be inline with what you or your husband interests. Factor in some down time as well during the summer months when he can spend time with friends and enjoy a less rigorous schedule.
 
If supervision is your concern, consider connecting him with a mentoring program, youth group or volunteering opportunity. The public library is a wonderful spot to begin the search. The attached web site will lead you to an article I wrote on the topic of planning for an engaging teen summer. Enjoy!
 
Regards,
Joe Bruzzese