At 150g he should be eating a medium to lg mouse. Roughly a mouse that is 15-20g. Any smaller and it will take him longer to poop bc he's using every bit of his meal and likely has nothing left over to poop out. They don't go very regularly though as others were Saying. It's only been 3 weeks and you are being a a worrier like we all have been at one point in time. You can up his feed intake or prey. Make sure he's well hydrated and his humidity is correct and he will poop for you soon. No need to try any liquid paraffin.
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