The hard part is over.
I'm matching you with one of our specialists who will be calling you in the next few minutes.
I agree that you need to get him out of the apartment and hire a professional cleaning service this week and make plans this week. Do not communicate with the board directly because it's gone beyond that. You need to work through an attorney.
It's often not possible for a social worker to speak with someone for a few minutes and determine what kind of issues they may have, if they have cognitive decline, depression, etc. I'd focus on getting him properly evaluated, which could be a challenge if he refuses. You might be able to use the pending legal action as leverage to get him properly evaluated. I'd try to find a geriatric primary doctor, as well as a neurologist who focuses on Alzheimers and dementia. A geriatric psychiatrist might also be a good option. The primary may be able to give you guidance, since your dad's inability to maintain his household is causing severe consequences.
I'm not sure if he has appointed a Durable POA or Healthcare POA, but, is he competent to do so now? If he's not, then I'd also ask the attorney about Guardianship. If someone is appointed Guardian, they can take over the decision making and deal with the legal matter that best protects dad, since he may not be capable of doing that now.
I'd ask the attorney what evidence you need, how the process works, etc. In some jurisdictions, a major factor in a competence proceeding is whether the person is able to properly operate their own household. It doesn't sound like he's able to do that.
I might use this as an opportunity to find a more suitable place for him. I'd also seek legal counsel and explore the options.
This might be a good opportunity. If dad has no idea what having a sanitary condo is, then, I'd be concerned that he's not capable of living alone. Maybe, he needs therapy, counseling or perhaps supervised care. So, I'm not sure if putting a band aid on the matter would help long term.
Deep messes that have attracted attention for years are going to be seriously bad as you reach the bottom.....even the air quality is probably bad for your health!
Make sure the HOA is fully aware of the plan...they will need to approve the dumpster to sit while it is filled.