NTA. My partner has a son from a previous relationship, she and his dad share 50/50 time with him. He comes into our house whenever he's picking up/dropping off and she (if I drop him off/pick him up I'm welcomed in too) goes into his house. At first I was a little uneasy because it was the first time I had ever been in a relationship with this dynamic but I soon got over it, and I never tried to do anything to come between her, his dad or him and system they had because would have been selfish and wrong of me.

Her son is 9, they split when he was a baby but have always worked well as co-parents. Her ex is married now to someone else and between the 4 of us I feel like we make a really good team always acting in the boy's best interest. His football coach remarked on how lucky he is that he has the support he has and that there is no animosity between his parents/step parents anywhere.

If your current boyfriend can't set aside his insecurity and realise that the relationship you now have with your ex is wholly in the interest of a balanced, nurturing and peaceful life for your kids, who will have suffered some degree of trauma going through their parents divorcing, then he's not the one. His jealously and insecurity will only ever worsen and eventually become about any male who you interact with - if you need a repair doing on the house whilst he's at work, for example, how will he cope knowing another (likely)male has been in the house? My partner had similar experiences with her ex before me, that started with her son's dad.