Welcome. I hope you find this as supportive as I have over the last 5-6 weeks.
I don't feel entirely comfortable giving advice on here, as I am new to my recovery myself.
But one thing that jumps out at me from your post is that you are judging your boyfriend on your dad's behaviour.
I know it's difficult to change a thought process that has been imbedded in you for so many years but I think it would help you if you could learn to seperate them and treating them as 2 different individuals/situations.
Ok your B/F has done some silly things in the past (most of us have) but it doesn't mean that we choose that life all the time. If he is 21 and just becoming legal for him to have a drink, then he probably just wants to initiate himself with his brothers. I know I got completely trashed at my 18, 19, 21, 22 etc etc etc but that's all it was and I soon grew out of it.
I can totally understand your worry but can you allow him to enjoy his b'day the way that he wants and set boundaries with him that still protect your feelings? I'm not sure, it's just a thought.
I'm sure you will receive some great advice on here and I hope you are feeling better soon xx