This is a tough one. Your son already has a relationship with his Dad and is now upset because he doesn't see him. When he saw him off and on he was upset because his visits were unpredictable. Either way he's upset.
I would need to know more about his bio-father and their relationship when he is around before I could know what was best. Based just on your information I'd suggest that seeing his bio-father hit and miss is marginally better than not seeing him at all. Especially if your son knows that you gave his Dad an ultimatum.
I've found that ultimatums rarely if ever get me what I want. You wanted him to be consistent. Instead he chose to disappear. I'd leave messages that you've changed your mind and that you want him to see his son. I'd let your son hear you saying that and then have him add to the message saying he wants to see his Dad. This helps to put the responsibility back on his Dad and off of you for whether or not he sees his Dad. It may not completely make things "right" in your son's mind because he can still believe that it was your ultimatum that started this.
I would also spend some quiet time with your son, playing a simple game or drawing or some craft and have a conversation about your concern for your son because his father has been inconsistent. Remind him of how disappointed he'd been when his father wasn't around. Do not put a negative spin on it! Just describe the facts, telling your son that this is just the way his Dad is and it's not his fault that Dad isn't around more often. Also that it's not your fault. Tell him you do want him to know his Dad. Sympathize with him about how much he misses him. Do not compare his bio-dad with your husband. Do not suggest Don can be his Dad. No matter what a great Dad Don is he cannot take the bio-Dad's place. They are two different people.
My 9 yo granddaughter has a bio-father that she rarely sees. She is old enough now that they exchange e-mails but I don't know how often. My granddaughter is in love with her bio-father even tho the rest of us don't really understand the dynamics. It's not because he's a great Dad. But he is her Dad and therefore important to her. She makes up stories about time she thinks she's spent with him. She saw him when she was 10 months old and "remembers" it. She saw him next near her 7th birthday and not again since. He hasn't sent cards or gifts for holidays, not even her birthday. But he is her Dad and she attributes all sorts of good times and her own characteristics to him. Her mother and I encourage that because he is a large part of who she is. He lives in AZ. We live in OR which does help explain the lack of presence to her.
My daughter has remarried and my granddaughter calls him Papa. He's a wonderful Papa . My son in lawl is as important to my granddaughter as her birth father is. He's her actual father figure and model. I suspect my granddaughter bases much of what she believes her bio father is on how her step-father is. It's working for her now.
Some day she may discover that her bio father isn't so wonderful but she'll discover that on her own. In the meantime she is thriving knowing that her birth father, Daddy, loves her even tho he's not around. She is also thriving because she feels the love of her Papa. Children can never have too many loving adults in their life. It doesn't matter if the one is more of a fantasy than the other.
If you are able to get your son's birth father back in the picture, even just a little, your son may eventually be able to stop longing for him because he does have a Dad in Don. If his birth father isn't around and your son believes it's because you told him not to be then he will continue to yearn for him.
At 7 your son probably understands more than you realize. My daughter has talked openly with her daughter about her birth father. She does not bad mouth him but she does say, he's just not very consistent and it's not because of you. That's just the way he is. He loves you very much. My daughter also calls the birth father and puts Monet on the phone. Her birth father does not call her. It doesn't matter who initiates the calls. It's important that the child is able to talk with their birth father if it's possible. Have some, even if it's limited, contact allows the child to be a part of ongoing history. The child is a part of the father. That doesn't mean he'll absorb the negative behavior. In fact he's more likely to reject the negative behavior when he's experienced it. Otherwise he hangs onto an unrealistic fantasy and feels anger.
My granddaughter's fantasy about her birth father is idealized but she experiences the reality of his inconsistencies. She also feels his love because we encourage her to experience that and we stay away, for the most part, from the negative side of his character We acknowledge the negative when she's expressing disappointment but we don't judge it. It is just the way it is. We would like to say, "he's a flake." but we don't. We sympathize with her feelings and tell her that even tho he didn't send a card we know he does love her. He just isn't the card sending sort.
I do believe that he loves her. Otherwise I wouldn't say he does. I think that since your son's birth father has spent some time with him he does love him. He just doesn't show it in ways that we expect..