You have to accept that yout fiance is a ‘batsu-ichi’ (once divorced) and has a history before you which includes kids. Being jealous about something you have no control over is a waste of energy, and its only natural that he will have feelings for his kids. You are not really competing with his kids as they are out of the picture. Its much harder when they are living with you and you are competing with them for his attention.
To the son you will always be the stepmom or ‘substitute’ mother though he has his own birth mother. He would have to deal with the fact that his parents have divorced and remarried to other people.
BTW on a Japanese divorce paper there is a line where parents have to choose who gets custody. In 90% of the cases its the mother unless the husband can prove his wife is an unfit mother. He cant see his kids unless his ex allows it and in a majority of cases its a complete split in the event of a divorce.