When is it time to go to the vet?

My usual vet is closed right now (Sunday afternoon), so the options are wait until tomorrow or emergency vet today.

My dog (8 months old) woke up today and wouldn't eat. Took him outside and he peed normally, then pooped twice (abnormal), but neither time was diarrhea or weird in any way. He proceeded to eat some grass before going inside, still refusing both kibble and treats. He was playful, maybe less than usual but not low energy, and he also started humping me a lot, but not his other owner. He was also gassy and we could hear stomach noises, but he did not react to having his stomach touched.

He stayed up for a while, was offered food (and refused it), and then let back outside. He ate grass until he vomited, a white, frothy vomit with grass in it. He then proceeded to have diarrhea once. He stayed up for a while longer, still refusing food but drinking water (especially right after he vomited), then was put in his crate to nap for 2 hours (will not nap outside of it). He was let out after 2 hours and immediately taken outside, but has not pooped - likely due to the fact our neighbor is outside making a ton of noise and it scared him right back in. He dry heaved a bit, but didn't vomit. Is still not eating, but is drinking some. Obsessed with jumping at and humping me, with sime mild playing, but not super active (but also not super lethargic).

He hasn't gotten into the trash, or anything like that, but it has recently rained a ton and he could've drank a small amount from a puddle. Otherwise, no idea of anything that would cause this.

When do we need to go to the vet? Should we wait until tomorrow? Is there anything we can do now to help him regain his appetite? Trying to decide if this is wait-it-out level of sickness or go-to-vet-soon sickness.