How long have you been with your dp and how long has he been living with you?
Your sons are at an age where they become nightmares to be around, tbh, so some of their antagonism will be teenage stuff.
I wonder if the rest of it is down to rebelling against a father figure they resent? How is their relationship with them? Does he discipline them?
They may also just not enjoy living with him. They might want more time with you 1 on 1. Perhaps they feel they've lost you a bit or can't talk to you, do have time with them without your dp? How do you spend it? It might also be that they can't come to you with grumbles or annoyances about him because you don't listen? Maybe you're always quick to defend him and they don't feel listened to? Their complaints wouldn't have to be legitimate in terms of your dp actively doing something wrong but it's not easy living with others and if you don't listen then how can I compromise be reached?
There is also the possibility that he behaves differently with them when you're not around. Don't dismiss the idea too quickly, it's common, even in the 'lovliest' of people.
I'm theorising a lot here but they're questions to consider. I'd also suggest spending time with just them and listening to why they're so unhappy atm. Without judgement.
I agree that if they're uncomfortable with his drinking then he could cut back from 2-3 to 1-2 and perhaps not every night.