Hello Francesca, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this problem which Oscar is having but hope that he's one of the lucky dogs who responds to medical management (which I assume would be steroids).
I wouldn't worry too much about no stool production for two days since this actually can be fairly common problem with dogs who have spinal issues.
What I typically recommend in these cases would be stool softeners. Options to consider would be as follows:
1. Canned pumpkin (NOT the pie mix); 1 tbs for large dogs.Squash can be substituted for pumpkin.
2. Human, over the counter Miralax 1/4-1/2th tsp twice daily
3. Raw bran flakes (wheat bran, not the cereal) at a dose of ¼-1/2 tsp daily
4. Docusate Sodium (Colace) 50 mg 1-2 times daily
5. Psyllium (Metamucil®): Doses vary and should be adjusted up or down depending on stool character. One teaspoon (tsp) of metamucil per 5 Kg per feeding mixed with food every 12-24 hours.
I probably wouldn't give him all of the above at the same time but I might mix and match depending on what seems to work best for him.
I hope this helps and that he will recover. Deb