If nothing has changed with his environment and foods and this continues then there's always a possibility of parasites... If you want, bring me a sample and we can do a fecal smear/float and see if we can spot anything abnormal . I'm off work this whole week .
Ditto on the silkworms causing the change. If you don't like the poop change you can slowly cut back on them and see what the tipping point is. I've done that with a couple of my males. Some can only handle one big silkworm, some two and some could eat ten and not have an issue.