Not only is it perfectly good English, which is often used for emphasis, but they are not necessarily redundant as "ever" doesn't mean "at any point in the past," and nor does "before." I am sure that you have heard someone somewhere use the terms "ever after" or "for ever" which mean the complete opposite. Of course, the meaning of "ever" is often derived from context, as in "I don't think that has ever happened," which is clearly in the past, and "I don't think that will ever happen," which is clearly in the future. However, "stronger than ever," has no clear indication and, while the reader can typically figure out that you probably mean the past, does not clearly indicate whether ever in the past or ever in the future and could, indeed, mean both.
"Ever before" is also different from "before" in that just using "before" doesn't necessarily tell us how long before. He could have been very strong, then weak, and now stronger than "before" but not "ever before." One could easily see that sort of statement being used about someone recovering from an illness. He might only be stronger that "just before," for all we know.
Therefore it is necessary to use "ever before" both for emphasis and to correctly specify exactly what you mean.
(1) "He is stronger than ever" (2) "He is stronger than before" (3) "He is stronger than ever before"
Are all fine.
"He is ever stronger than before"
Doesn't make sense because if you look at (1) you see that "ever" is working like a unit of time. The sentence is of the same form of as "He is stronger than yesterday", but instead of yesterday we have "ever", which semantically means the same as "forever". So your sentence is equally nonsensical as saying "He is yesterday stronger than before".
Perhaps you're thinking of "He is even stronger than before" - which does make sense but means something different.
It doesn't make sense in English. The sentence implies that the guy is "more strong" than he has ever been. Let's pretend strength is measured by how much he can bench press. Previous to this sentence, the guy could only bench press 100 lbs. Now, he can bench press 120 lbs. A new personal record. You would say "he is stronger than ever before". If you said "he is stronger as ever before" it almost contradicts itself. "He is stronger" means he's never been this strong before now. "as ever before" means (in this sentence) "as he has ever been". So if the guy's personal best was bench pressing 100 lbs, then he was in a car accident and was seriously injured reducing his ability to only being able to bench press 50 lbs. After years of training and rehabilitation, he finally gets back into the shape he used to be in and can now bench press 100 lbs again. Then saying "he is as strong as ever before" would work, but still not "he is stronger as ever before". I hope that helps and my little story didn't confuse you even more!