Ah man, this is complicated over all, it's hard to say what it might be, sometimes they just go through phases like this. I would try things (such as not using lotion for a day or two or changing/removing jewelry, etc.) but this could also just be a phase of testing boundaries ("how hard can I bite and what happens when I do?"), or it could be something about how you react that he's looking to make you do. Do you yell pretty loudly when he does it? Maybe he enjoy the fun noise. Do you give him a toy right after to distract him? Maybe he thinks it's a reward? Or that it's just a good way to get your attention. Those little guys just ALWAYS want attention and they hate when you're not looking at them. Could be that something is scaring him and he's trying to warn you, Curtis used to get super aggressive if we took him in the kitchen, he bit me on the nose and neck and hands trying to make me leave the "bad room" since he knew it was a room he wasn't supposed to go in.
My best advice is to have a decently thick dish towel on hand to scoop him up gently and safely and place him back into his cage when he gets aggressive. You try your best not to yell (though I know how hard that is, it hurts so much!) you scoop him in the towel so that he's only able to bite at the towel, then place him in his cage for about 5 minutes, in that time you ignore him, possibly even leave the room, and after about 4-5 minutes, when he's calmed down, you come back over and let him out. Repeat when necessary and be focused on showing praise and attention for good, happier/friendly behavior. Try to rewards him with scratches on his head or attention when he nibbles lightly for your attention rather than chomping down hard. Heap lots of love and sweet words on him when he's being a good boy or doing any behavior you like.
And beyond that I wanted to say, sometimes it's not even something YOU did wrong, sometimes parrotlets are just moody. It could be from itchy pin feathers, lack of sleep, or just a bad mood, but sometimes there would be months of a nice, sweet, little bird, and then out of the blue he would get very aggressive again. Try your best to ride it out and to forgive him. They can be little bullies sometimes.