Hello, I'm Dr. Deb and will do my best to help you today.
I'm so sorry that Peanut had an ototoxic reaction to the medication which was dispensed for him. While this isn't a commonly seen reaction, it can happen in some patients, unfortunately.
There are many reported cases in which hearing returned with prompt treatment such as I'll outline below. While there's no guarantee that he will regain hearing, it's not a foregone conclusion that he won't. As to the timing, it might take a few days or weeks to know if the following will be successful.
1. Melatonin 5 mg Just be sure to double check the ingredient list and DO NOT purchase one with Zylitol in it (which can be toxic to dogs).
2. Oral alpha lipoic acid 100-200mg/ day along with acetyl-l-carnitinine (200mg).
3. SAM-e 200mg a day
4. Vitamin E 400 IU once a day
5. B-complex ( B-50)once a day
6. 7 days of oral dexamethasone (not prednisone)
Most, if not all, of these supplements are available online or perhaps at human health stores. #6 would have to be dispensed by your vet.
I hope this helps and that he's one of the lucky ones whose hearing returns. Deb