I don't want my boyfriend to talk to his mother
Why would you want to keep your boyfriend from talking to his mother?
She is one of the reasons why he is suffering from depression.
I understand wanting to protect him, but you should discuss this with him before doing anything. Make sure that she really is one of the sources for his depression.
We already talked about it.
While she may be a source of his trouble, that may be because he needs to resolve some conflict with her. Talk to your boyfriend and figure out why he is not a fan of his mother. You may be able to help resolve one of his personal issues.