It is going to be o.k.
Don't take this as a flame, as it is being spoken as gently as it can be.
In my honest opinion, though, you are blowing this way out of proportion. Yes, you say he is financially independent, but being under his parents' roof is not completely independent. He is still strongly tied to his parents. They pay for his lodging, food, electricity, phone, fuel, etc.
There are a lot of parents who feel that if their child is living under their roof, the child must live by their rules. This may be one of those cases.
Also, did the mother not allow him to quit, or did she strongly suggest he not quit - or suggest he wait to quit until a better time? Do you think your boyfriend is taking her advice, but blaming it on his mother because he knows you will not like it?
Take a deep breath and re evaluate when you have some time to think about it. It's not the end of the world or the end of your relationship because he did not go with your plan. It doesn't seem to me like it affects you or your relationship except that it is not what you wanted him to do.
Denae