rubbish - you are an adult, and you did decide to get drunk. However, he decided to stay sober, help you get drunk in order to sleep with you, and then blame it on you the next day. What a complete (I'm sorry, I swear occasionally on these threads, but..) what a complete cunt.
I have friends who are young enough to be my daughter, we socialise, we drink beer, I walk them home at the end of the evening. Sometimes they're trolleyed and get a bit frisky, and I make sure they're in their front door and safe home, then I go home alone. He completely took advantage of you here.
If I were you, I'd tell your mum 'yes, you're right, we went out for dinner, he got me paralytically drunk to the point of amnesia, whilst staying completely sober himself, then he drove me back to mine instead of yours and then slept with me. I regret it hugely, I'm horribly embarrassed and I could do with never seeing him again, okay?'
Although my mum probably wouldn't take it well, with the implied homoerotic angle and all, in my case.