Trust Fund or Not?

So a little back story. My fiancé whom I have been with for 8 years and have two children together has another child from a previous relationship. Without going into too much detail, he was in jail when his ex gave birth to their child. When he got out of jail his child was almost 4 months old, the mother of his child was dating and living with his best friend and a relationship between the child and his best friend had already been established. Fresh out of jail as a convicted felon, he knew he was in no place to be able to truly provide for his son and give him stability at that time. He made the decision to step aside and allow his best friend who was now dating the mother of his child to fulfill that role of dad. This wasn’t an easy decision for him to make and he felt in his heart that was the best choice for his son. His ex of course holds animosity towards him, which I understand her point of view as that’s really difficult for a mother to swallow and understand how he could walk away from his child. But at the same time I can also see his perspective in the sense he knew he was in no position to give what his child deserved and didn’t want to disrupt the family dynamic his ex and his best friend had created with his son.

Fast forward to now, him and I have two children together and his ex with the child ended up marrying his best friend 6 months after. They have two other children between them and there has been no contact.

Present day, our family is about to come into a large sum of money and we have discussed setting up trust funds for our two children but we also have been discussing potentially setting one up for his oldest child. But we aren’t sure if this is out of pocket or would be perceived wrong. Should we continue no contact and allow each family to live separate lives? Is it wrong to set up a trust fund for his oldest even though he has no idea who we are? I’ve spoke with the mother about a year ago and she said they plan to tell the child about his biological dad when he is 17/18 so he will eventually know that he has a different father than the one who raised him and who has been his dad his entire life. I honestly just want to do the right thing and I’m not sure if this is just a bad idea and we should let it go. Or even just set up a savings account meant for him if he ever decides to reach out after learning of his biological father. I truly don’t want to cause any riffs or trouble. I just want to do what’s best for the child in the long run.