He is a fin biter, which means you can either try to find out why he is biting his fins and help him stop, or just let him do his thing (which tends to be what they do anyways).
He looks to be a delta or a halfmoon, which are known to be big fin biters- it is a side effect to creating large fins on fish. They feel the drag/weight of the fins and bite them off to be able to swim easier. If the filter isn't baffled and is giving off a fair amount of current then he could be biting them because of how hard it is to swim with large fins and a current.
He also may be stressed due to water changes, temp, too much light, etc. It varies what sets them off- I have an old male veiltail that will shred his fins if he can not see another fish for any length of time. He has to be in a divided tank, or in a tank next to another otherwise he would have no fins left. Even though they are territorial, they don't really like to be left alone for a long time- so make sure he is in a room that people are in for a good chunk of the day as they do pay attention to us humans all the time. Show him a mirror a couple times a day for a few minutes each time.. if he thinks there is another male nearby he may stop chewing his fins in order to grow them out to try to keep the other male at bay.
Are you asking anything you can use, or are you seriously wanting someone to "fix" it?
There is nothing chemical that will help him, other than make sure to keep the water clean and a good diet full of protein.
As far as having someone else help him.. you can do anything anyone else can do.
Just remember that all bettas are different.. and people keep them in all different ways. He is your pet, he isn't for showing or breeding.. yes, you want a nice pretty fish with big fins but know that he still has a big personality, and his fins will be all different sizes throughout his life depending upon what he does to them. It's only been three weeks, give him time to adjust more to his new home and once he gets settled in he may stop. He is still a beautiful fish, he just has a "quirk".