Hello,
Well, I don't have a Uro, but know several people who do. They recommend play sand, quartz sand, a sand & clay mix, or a sand-clay & gravel mix. That seems to be the preferred one because they are built very sturdy & like to dig & burrow as you know.
As far as the mouth rot, good grief!! Does your boyfriend forget to look over him?

( That needs to be taken care of before it becomes a systemic infection. Get some betadine & dilute it & dip it in a swab to clean out the mouth really well. Get the debris & any dead tissue swabbed out so it doesn't sit there & rot more. Get some nolvasan cream to put on it after cleaning & you can use a swab for that also.
Are his temps up to par & is he keeping him dry too? He must have injured his mouth on the millet possibly, I don't know. If that mouthrot goes too far, it will kill him eventually.
Let me know how he is doing.
Tracie