Could be the change from the mice to rats - a temporary issue that may resolve itself. But I have never seen such problems in my snakes from changing food items.
Other things that can cause diarrhea are internal parasites. If the crap has a really sour, foul smell one of the signs that something is up. If you have some flagyl you could try give him some of that, it could help. Or take a faecal sample to your vet & he can test it for presence of worms or other micro-bacteria.
It is really easy to type suggestions here, but make sure you check with your vet before giving your snake any kind of medication. Treating with wrong medications can have just as bad effect on them as the original problem. But in saying this, worming your snake shouldn't require a vet trip. ou can pick up worm medicines from the pet shop.
Flagyl is only available from vet with consultation of your animal but I have found it to be fantastic stuff for upset tummies. It will kill other parasites that worm medicines won't. My beardy bounced back from sad, grey and ill looking health within hours of receiving one dose of flagyl.
