When my first was born 4 years ago, he also had the same problem. The doctor told us not to worry about and it would be ok when he is 1 year old. His 肚臍 turned normal before his 1st birthday. Now my daughter was born on May 23, and I also found her 肚臍 is 凸. This time I don't feel worry at all.
You don't need to do anything, it will become normal itself.