Wow that's a tough situation. I'm really sorry that he misunderstood the situation. When he said that he didn't want to see the message and you don't have to explain anything to him, does that mean that he is mad and avoiding the situation, or is he saying he trusts you and doesn't need you to prove it? If he is angry, then perhaps give him a chance to cool down. Is there any way you could get a family member to confirm you have a foster bro? If he knows that is true he would probably understand why you said you didn't know whose number it was.

Trust is an essential part of a relationship. It has to be mutual. If you two don't trust each other I think you should ask yourself why. If you can't learn to trust each other the relationship will be hard to maintain.