4 month old won’t sleep

My bub is almost 21 weeks, lately he has been rejecting his naps with me (he’ll sometimes sleep for gramma) and he has been a nightmare at bedtime. He started his sleep regression a month ago and is still sleeping horribly, hour stretches at a time, waking up screaming, only consoled by the boob etc. Around 5/6 pm he absolutely loses it, it’s obvious he’s super tired but he won’t sleep, it’s like he has forgotten how to sleep. I can hold him, rock him, feed him, and he still screams until he passes out from exhaustion. I have no idea what’s going on, he’s never been the best sleeper but he used to take 3 naps during the day and have one 4 hour stretch at night. He’s definitely started teething, we switched him out of the bassinet and into the pack and play almost 2 weeks ago and I went back to work; maybe too much change too fast? Any words of wisdom?