Complete solidarity with you. I lived with and cared for my grandmother throughout her Dementia until she passed away. Every day was a roller coaster. I really would contact Social Services. The only problem with them is that their assessments last an hour and as you all know, an hour is no where near enough to really see the reality of the person's illness so you must be persistent. They can put you in touch with fantastic organisations that can offer you daily morning carers, cleaners, a carer who comes for a morning or afternoon once or twice a week as company for your dad and respite for you for a few hours. They also have carers who can come of an evening if you'd like to go out. They really are invaluable. Through them, we even found a care home who offered part time respite which we couldn't have lived without. My grandmother would go to stay there for a weekend of a week and the interaction was fab for her and the break was amazing for us. Gave us a chance to recharge. We were also put in touch with charities that ran workshops on coping mechanisms and things we could do to help her. The Alzheimer's Society run day sessions for people like your dad in which they do a range of activities such as light physio, singing old songs from their era, dancing etc. My dad now volunteers at one and can't speak highly enough of it.
I can't lie to you, this won't be easy. There will be days when you want to scream at him but you must not feel guilty. We ALL go through this. There will be days where he will vent at you, but you must remember that he is just frustrated because he will sometimes be aware that there is something wrong with him. There will also be nicer times though. Times when you can get him talking about old times and it will freak you out how much he seems like his old self in those moments. With the right care and support, this can be okay. You're already doing a brave and very noble thing. Never forget that. Even if he doesn't thank you directly, he will be so thankful and full of pride. My grandmother would always say this to other people but never to us! Sending you lots of love and strength xx