Did you take him to a Certified Avian Vet (not just an exotics vet that sees birds)?
I don't have Lorikeet experience, but if you got him when he was a baby and now he is growing up, then the behavior change could be due (in part) to that. They mature sexually around 2 (according to Google) but hormones/puberty kick in before then. If he is hormonal, that can lead to aggression, screaming, cage-territory issues etc. Make sure he has no access to shadowy spaces (e.g., boxes, shelving, under clothing, pillows, under furniture, SNUGGLE HUTS/TENTS) and make sure there is no nesting/bedding material in/around his cage. If you do pet him, stick to the head and neck only, as any other touching can lead to heightened hormone stimulation.
Sleep also plays a huge role in regulating hormones and immune function, so make sure he is getting 12 hours of sleep a night (quiet sleep in a dark area). Do not ever cover his cage during the day unless it is time for his 12 hours of sleep. Some birds can do okay with 10, but 12 is the average for most species--- other members can confirm whether 12 is the average for lorikeets.
Even if he isn't hormonal, the stuff above is still important to keep in mind because you don't want to start things with babies that won't be acceptable when they reach maturity.
Was there anything that might have scared him around the time when the behavior changed (a move, a vacation, new furniture, a new person in the house or the absence of someone familiar, a change in routine, major change in lighting etc)?
I am guessing the vet did blood work---if not, assuming it can be done safely, I would suggest it.
Where is he plucking and when does he tend to pluck? Is he actively removing full feathers or just chewing/barbering them down to nubs?
It is probably hormonal, BUT,
did the vet do any sort of culture of his crop or swab the area he is plucking? That is another thing to consider if he didn't.
The last medical thing that you might consider is an xray to see if he could possibly have some sort of enlarged organ or a blockage (again depending on where he is plucking etc etc)---I am not a vet, but a CAV is going to be way better about running tests and getting to the bottom of things than an exotics vet.
On this list, I would say blood-work would be the most essential.
You might want to check the humidity levels in your home---that can sometimes do it. They say to shoot for 50% humidity...
How often does he bathe?
Make sure you are not using any products on your skin or in your home that could be causing irritation (hand lotions, nicotine, chemical cleaners, basically--anything with a scent).
Make sure his diet is balanced/appropriate for his species--I know they drink nectar, but I don't know enough to make suggestions on the nutritional end of things.
How big is his cage and where is it located (is it in the main hub of the house or tucked away in a room? Is it in a room with natural light or not?
How much time out of his cage is he getting daily?
Does he have lots of toys to keep him busy (ones that he actually likes)?
Plucking is tough which is why I am asking so many questions because it can be caused by so many things.