Is it sleep regression? teething? something else?!

So my son is just over 5 months old. He's been sleeping through the night since around 2.5 months. I'm also usually able to lay him down awake for naps and at night and he will fall asleep on his own. Now all of a sudden the last week-ish he's been fighting naps and getting super fussy. At night he fusses a lot and most of the time with his eyes closed because he's so tired but he's fighting it. He also usually sleeps until around 9 and he's been waking up at 6:30 and refusing to go back to sleep even though he doesn't fall asleep until around 12. I just recently started feeding him cereal so I don't think he's waking up because he's hungry. I thought maybe teething, but I don't see anything coming through, then I heard of sleep regression.. idk if I should let him fuss it out or if there's something really bothering him. Help, we're tired 😭