Ok...Lemme see if I understand what you've said....
When your bf leaves during his tangent, he goes and stays with his mother. And while he is at his mother's place, the mother of his child (his ex) comes over to stay with him, as well? Is that right?

If so, you have his brother and his gf's verification that what his mother is stating to you is a lie, which is exactly what your bf is telling you, (that his mother is lying)...correct?
If that is the case, (that his mother is, in fact, lying to you), then where is the distrust your bf has earned with you?
I fail to see, (or possibly don't understand) where your bf has earned your distrust.
Granted, his behavior of his temper tantrums are uncalled for, especially repeatedly. Maybe if you can get to the understanding of why he continues to behave this way, then you may be able to resolve something with him so he wont have to storm out every couple weeks?

I'm not even certain if I'm understanding this correctly.
Maybe if you can provide a little more information/clarity, then we can be of more help?

Shangrala