Maybe when he picks a fight, ignore it and say to him I know you have noticed me growing and changing, just because I have new hobbies and interests it doesn't mean I am leaving you. I know it will take time for us both to adjust to my growth and change, but it will be okay, we just have to be gentle with each other.
Can you let him be with his fear and scared feelings? And maybe reassure him that things will be okay. You know something along the lines of it will be okay, things will be okay. I am not going anywhere. I can be here whilst you are uncertain and unsure, it is okay, but I won't argue with you because I don't want to waste our time together.
Perhaps intensely engage him in something or really give him lots of attention? I don't know what works for him.
Maybe every time he picks a fight just say to him it will be okay. We are okay. You are okay. I am okay. He might need a fair bit of reassurance.
Maybe his arguing is a way of seeking reassurance from you or at least keeping a connection with you? So if that is so you can answer his criticisms in a different way.
Also if he is insecure and uncertain maybe get a few cards, write on them and leave them around the house or put them in his brief case, so he gets a message of love and reassurance from you randomly through the day.
Maybe he thinks if he is fighting with you he is connected to you more - who knows, we humans are a very strange lot and often we don't even know why we do what we do.
Just a few thoughts.
@Nicolette's chiming in is good, because maybe he needs to know you remember the good things he has done for you. Maybe bring up the past and say this was happening and you supported me, or bought me flowers, or made me laugh.
And catch him doing "good things" so if he does something, even if small, say thank you and comment on it. At the moment I am having troubles so I try to catch my partner B, doing good things and say "That was thoughtful!" "That was sweet!" "You were very good to this person!" - real genuine positive feedback.
Bringing up the past when a person has done some good things - well it can make someone feel heard and like you see them.
You don't really know what is going on with him - and you know what positive things that he responds to, google some more and experiment with "positive reinforcement". Whenever I see B doing something good I comment on it, so he knows I get him. He kind of smile happily and goes on his way. If I am not doing that then I try to get my mindset back to that.
This maybe be off base or helpful. I don't know, but a few ideas anyway.