Am I the problem?

My kiddo (5) is in his first year of kindergarten. He is the youngest in the class and we sent him early but the pre-k teachers as well as his dad and I thought he would do great, and he is but he is very hung up on one friend from pre-k who goes to a different school. A little back story… my son and this boy grew very close in his 4/5 year of pre-k. They walked in and out of school together every single day. Played on the playground together. And even played on the same T ball team. The mother and I were also very close texting and talking every single day, I considered her on of my best friends. Through the summer we had play dates at least 1x-2x a week. Flash forward to school starting and it’s crickets. They live very close to us… about a 2 minute drive but we somehow live in different school districts. My poor little guy asks every day after school “am I ever going to see … again?” Or “do you think … still wants to be my friend” it is breaking my heart!! The mom and I still talk but not nearly as much as we used to. But I ask to do something every other week and she ALWAYS makes an excuse as to why they can’t. “He’s sick” “we’re going to the football game” “going out to dinner” but doesn’t ever offer an alternative day or time. Do I throw in the towel on the friendship for my kiddo? I just feel so badly for him because he’s really missing his buddy but it doesn’t seem like the other mom wants to bother with it.