I'm an adoptive mum. DH and I have always been honest with our 2 DDs (birth siblings, now 9 and 6) about their history. When they were small, we spoke about their 'tummy mummy' and read their life story books with them (provided by their respective social workers).
We recently read one of two 'later life' letters to DD1, who was asking a lot of questions about her life. She responded well to that and it seems to have relieved her anxiety. She refers to her birth mum by her first name and knows that she wasn't able to look after her.
We took DD2 to meet the couple who fostered her for the first year of her life, and that meant a lot to her. She loved seeing the bedroom where she'd slept while she was with them.
Children need honesty from those who are bringing them up so that they know that they can trust them. They can cope better than you think. Obviously you shouldn't tell them all the gory details, but whatever answers you give, they need to be true. There's no such thing as 'white lies' in this context, even if the intentions are good.
As to whether you should tell your DS the truth, OP, I understand why it would be difficult, as he's at a tricky age. But the damage would be far greater if he heard it from someone else, so I think yes, you should tell him.
This isn't about the DP, it's not his business. But it's very much your DS's business, OP, and he needs to know the truth.