It is really frustrating not getting to be with the people that you care about. It's understandable that you are upset. I'm sorry things are so tough right now.
It is important to understand your mothers perspective though. At your age you have a lot less life experience than an someone older. Imagine how you felt when you were in first grade versus when you were in 5th grade. There's a lot of maturity that happens over the course of 4 years. So it's understandable that your mother would be concerned, maybe she had experiences or friends who went through similar things at your age. You may also want to check the legality of the relationship.
Hopefully you're not considering sex at such a young age, but depending on your state law, a sexual relationship between the two of you could be considered statutory rape.
You mentioned that your last relationship was unhealthy, what do you mean? Were you hurt?
If you really want to give him a chance, the best thing you can probably do is have him spend time with your mother. Maybe if she can see the way he behaves in her environment she will see what his true intentions are. Have you discussed having him come over and spending time with you and your family?
Boys Town Counselor PW