He could have had some slight damage to his lower GI tract or even to his anus. (I'm not saying permanent damage because I don't know)
But if any animal, human, fish, bird, dog, whatever, is under a constant laxative effect there will usually be some where and tear (and possibly other issues but this is just in relation to blood) that can cause "bleeding from the anus" (the blood may not be directly from the anus but slightly further up the GI but still fairly close if it looks bright red.
Also I use freeze dry but soak them before you give them to your betta to prevent it ballooning in his gut which can lead to problems eg constipation, intestinal blockage, electrolyte imbalances (because it expands by getting water from the betta's body), etc
Think eating dried pasta as a main meal for every meal - not pretty!
So you don't have to give up the freeze dry food, you could keep them as a back up, just make sure to feed them sparingly and when you do soak them before you give him one (that's another thing I only give one freeze dry worm at a time/per day).
Freeze dry can be good because it is the least likely food to carry disease/parasites. Where as live foods can carry diseases such as salmonella as well as fish diseases/parasites. So there are always pros and cons to the different options.
Also does he have plenty of hiding spots?
Silk plants would be good for your situation also a leaf hammock can be good if the fish feels too weak to always surface (and it is cute to watch them perch on it