Telling your child about bio dad

Hello! I'm mum of 2 amazing little boys and expecting a third boy, this october. I'm in stable relationship for the nearly 6 years with my husband, married for 1 year now. Our oldest son who turned 5 this year is not biologically his and we want to start talking about it as recently he is more curious about things and more aware if we explain something, we did not talk earlier because he was speech delayed and had trouble with understanding how things work. A little backstory, I was with my ex more than 6 years ago, when I got pregnant. We wanted a baby, but soon he changed, after moving together and me putting my soul into our place and into him, he started to "work" alot, never be home, always showing up without any money, leaving me home alone without anything. His mum was crazy person, she did not like me, she accused me of cheating, being with him because of the money (that he didn't even have) etc. Because of her I lost my job as she went to my boss and told stuff about me that wasn't true, so I was stressed non stop, my then ex wanted it to work out but never showed up, never was there, never helped. So I said no to him, I broke up, wanted to do nothing with him and a week later found out I was expecting. Because of me not being able to work, not having close family that helps, having his mum on my back plus him who was not interested at all I got homeless, pregnant and did not want to live. I've met my husband in my worse, he supported me, wanted to help me but best of it, he saw how desperate I was about abortion, he said- I know that it is hard and you think no one wants you or the baby, but I do, if he doesn't want it, I want it.. so ever since we've been together, we moved in, he took care of me, helped me regain my ground under my feet and best of all he was the father of this baby ever since I was 8 weeks pregnant. He took care of me while pregnant, pampered and loved me, talked to the baby, he was by my side when I gave birth, he helped to name the baby and took care of all his needs and he did want to be on the birth certificate, so I allowed to do so. The bio dad messaged me only once in pregnancy and mocked me, tried to give me 70$ to make up for me becoming homeless, left without food and alone pregnant, I did not take any of that, I had my mind set that if I turn back there shall be nothing for my child including attentive dad. After having our son, my husband never has even mentioned that he is not his, he loves him so much, you can see the happiness within his eyes when he comes home from work and has the sweetest cuddles with him. He is the best dad there ever is to my child and I don't regret making the decision I did, as I can give him all the love and experience of having a loving family. We have now another son aged 4 together and expecting our third son, he is so excited for it. We got married last summer and both our sons guided me to altar, because I have no father and no important male family that could take the role. I can't be more happier. So from the begining now, I have started thinking on what would be the best age to start to talk about this, even tho I don't want to, I have to tell him, so he is aware and understands that it wont change a thing, as we both love them all equally as much and he will always be my husbands son, nothing will change. I just don't want to keep him in the dark. How did you start talking about it with your child/ren? Thank you!