Hmm, I did some reading and looking around, there have been some cases where geckos will have a little runny poop after eating a butter worm, all though not all have but some have, or that he may be having these weird poops because the change of diet and that it should return normal soon, but if it does not I urge you to take a visit to the vet then
Has he had any sort of loss of appetite since the runny poops? If he is having runny poops and losing appetite then that's a pretty strong indicator of a parasite, which then again, can be confirmed through a fecal