He may have started liking your posts on Instagram in September until mid November because during that time, he may not have had any good dating prospects.
The reason why he ghosted you in the first place after having gone out on dates with you for four months may have been because someone better came along.
Being that you both matched on Tinder, there is a strong likelihood that he ultimately matched with someone else that he preferred over you.
His time with that person didn’t last long and he got back in the hunt.
This was the period that he most likely started liking your posts on Instagram.
He wasn’t ready to take a big step in actually contacting you because he wanted to see how you would respond to his likes on your Instagram.
He was most likely trying to get a read on whether you still liked him.
He has since stopped liking anything that you post because he has most likely matched with other girls, most likely on Tinder, that he wants to pursue.
Hence, you are not a priority to him.
If his experiences with those new matches don’t work out, he may end up on your Instagram once again, liking your posts.