You said the flush "went well" but then that he had his "first bowel movement" four days later -- did he pass any stones over the weekend?
Some people have passed white stones:
I've come across posts where people pass what they think might be candida, or mucus, or worms -- any of those could be white.
The problem with symptoms is that any particular symptom could have a number of different causes.
Weakness could be sign of electrolyte imbalance for example.
Or it could be that going that long without a bowel movement permitted re-absorption of toxins that he'd have been better off passing. You might want to look into treatments for constipation if this is a frequent thing with him.