Please let us know how the surgery goes.
Remember: After surgery he will be on antibiotics. Antibiotics kill all bacteria, even the good bacteria. The good bacteria is often referred to as "flora", this flora/good bacteria is what digest your little one's food and keeps his body functions working properly. (Extremely important for a chinchilla ... when a chin's digestive system shuts down, so does he. The first signs of this is small hard poops and much less in number ... try not to let it get to this point.) To keep/replenish the flora/good bacteria, live culture yogurt is good. BUT ... yogurt is a milk product and milk products counteract the antibiotic. Soooooo
to make sure both the antibiotic and the yogurt works properly, give the yogurt half-way-between [at least 3 hours apart] each dose of antibiotic ... this way both can do the jobs they need to do for your little chinchilla.
Hint: Yogurt is an acquired taste for some, you might want to let him start on it now, so there will not be so many different things/changes going on when he is feeling so rough after his surgery. I find most of mine like strawberry and/or blueberry best.
Some chinchillas love it from the beginning (I have one that cries for it), while most take a little while getting use to it, and some will never take it. (If you touch a tiny taste of it to his lips and he twirls round and round spitting and wiping his little mouth ...
... forget it ... he will probably never develop a taste for it. ) Should this happen check at your local nature/natural store (like GNC) and get him some acidophilous to add to his food several times a day. Acidophilous and food containing it, has to be refrigerated.
Jo Ann