Hello, I am Rafael. Thanks for asking your question - I'm here to support you. (Information posted here is not private or confidential but public).
I am sorry to know about this frustrating situation.
You were this close friends but got distant because of these school changes. Now you got a new chance to reconnect but you feel afraid it is not reciprocal since he doe snto show the same interest as he had before, right?
You know he has changed, but you do not know why, then it is wise for you not to push him, since it could get further distance between you.
Now you want to know how to stop obsessing about him. I'd say that first you need to fully acknowledge what you feel, without any denial or avoidance. Then be mindful that allowing yourself to obsess would only undermine your chances to get back your friendship, which would never be the same as before, since both of you have changed. Then keep an open mind while being fully realistic, since you do not know how willing he is to work on having this close friendship as you had in the past.
You could have positive expectations about it, just to confront your self when finding any obsessing pattern perturbing you. You would identify the tendency, stop it and redirect your mind and actions towards something healthy, productive and enjoyable as possible. Don't try to deny or repress what you feel, just acknowledge it and willingly redirect yourself towards something that actually help you to feel and be in a better place.
Set clear boundaries and limits, for example making a commitment not to contact him more than 2 times a week, unless he takes the initiative and contacts you. DO not isolate nor stay passive thinking too much about it, but get busy shaping and enjoying a healthy life without limiting it to this person or your expectations about your friendship, since that is something you cannot control.
Does it make sense?
It does make sense, thank you. But how can I ask him if he wants to work towards what we had?
Sure you can. You can and should take that initiative consistent with your expectation. I would not guaranty he would be assertive or even welcome your invitation, but that's something that does not depend on you.