Sorry, it really doesn't make any sense in speech though there isn't any grammar mistake in there.First,you need to think about what idea you wanna to express to other people and make them understood.The main problem of this sentence is that you usethis phrase "But for".If I were u, I would use "except" instead of "but for".
His parents is his relatives and will surely help their son in this situation while others won't.So, the whole meaning becomes the fact that only his parents will defend the suspect.This meaning complies with the real situation in real life.
"But for" gives others the idea that if his parents are't there or if he has no parents.
Then, people get confused about what you want to express.
Both writing and speech are to get your idea across,aren't they?