Why don't guys keep trying to impress their girlfriends in a relationships and stop putting effort into their appearance?

My boyfriend was very attractive and handsome when we first met and when we started dating, in the first 4 months of an official relationship. He had amazing, gorgeous, not-too-short and not-too-long hair that had shape, style, softness, he looked nice. His outfits were planned and intentional, he was stylish. He was always clean shaven - my dream guy. Checked every box. Later down the line, though, he seemed to get complacent. He stopped doing that. He stopped showering before seeing me, would let his face get scruffy and his facial hair would irritate my skin and make it red and badly itchy, wore pajama pants every time I saw him. He completely let himself go, drank too much and got a beer belly it will take a LOT to get rid of, he hasn't gotten a haircut in over a year now and he likes his hair long because he thinks it's badass and wants to grow it to Peter Steele length, but it looks so ridiculous. He looks greasy, dirty, slovenly... his hair strands stick to his forehead or the top looks mashed down and he puts it in a ponytail like a girl, I have to part his hair like curtains to kiss him. It's not an attractive look at all and I find myself turned off and desiring him physically less. I tried gently persuading him to get his hair trimmed and shaped up, nothing dramatic. And he ALMOST listened to me, but he has a DUI so he asked his parents for a ride to the salon and they said no... because they like his hair long. It reminds his mom of her brothers and his DAD when they first met. ummm... a mother doesn't need to be aesthetically attracted to her son, red flag. So her opinion trumps mine? When you love someone, you want them to present as the best version of themselves and he's just not the guy I fell in love with. He's changed, and it's like he's lost all his confidence and self-respect. He seems different, like a shell of himself. What's going on? Anything I can do, and what can save this relationship? He's otherwise a loyal, committed, sweet guy.

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Should I just give it to him straight, reassure him that I love him but his hair looks greasy and sloppy and like he doesn't take care of himself and it's giving me the ick, and that I thought he looked so handsome and dreamy when we first met and started dating and that was his best look? That I think he'd feel a lot more confident if he got it shaped up a little, maybe some layers? When you look good, you feel good. Should I just be brutally honest at the risk of hurting his feelings?
Why don't guys keep trying to impress their girlfriends in a relationships and stop putting effort into their appearance?
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