Cat is drooling thick saliva/mucus, looks like he is grimacing

We have a male, 2-year-old, neutered, vaccinated, completely indoor only cat that started grimacing and drooling this thick, sort of slimy saliva (?) and is constantly in a crouched position as seen in the photos: https://imgur.com/a/J4d6xLQ

He ate all of his usual wet food + kibble mixture meal this morning with his older brother (4 year old cat, also neutered, vaxxed, indoor only, who is completely fine). For context, our two cats are very friendly and like to play with each other, share spaces and resources, eat/sleep with together, etc.

He was fine and his usual self this morning, but my boyfriend noticed that he did not look good later this evening and saw the saliva and drool all over his mouth.

Won’t eat any kibble, wet food, treats, or Churus (big red flag because he will go crazy for them) when presented to him, won’t drink any water.

Bf also says he noticed that he didn’t eat his last kibble mini-meal (of which both cats are fed the same portion from an auto-feeder every 4 hours, 4 times a day), but didn’t think too much of it. We’re both busy during the day and in and out of the apartment, so we couldn’t have monitored him earlier. He never likes to get to small pieces of plastic or random little things lying around, which is ironic because our other cat absolutely loves that stuff.

However, he will play with and chase his favorite toys, a wand toy and a laser pointer, just not as energetically and wildly as he usually does. He’s not really lethargic, he’s still kind of walking around and being a cat, but he’s just always resting in this pained-looking crouching position.

I took a quick look at the litter boxes the cats share, and it seems fine and normal.

We combed through the apartment looking in all nooks and crannies for any vomit or evidence of him eating or getting to something, but we couldn’t find anything. We even turned off all the lights and walked around with a blacklight and still found nothing. Only evidence of the drool in his favorite spots.