Having autism and Attention deficit disorder making friend and maintaining them is difficult. If your friend is not on the spectrum their is a chance he has just out grown you because we develop mentally and emotionally at a slower rate than the typical person.
For example I'm twenty four and mentally because of my issues i would say mentally i am around sixteen mentally and emotionally i might catch up with the rest of my peers of the same age or i will constantly lag behind them and that is okay. most of my friends from high school i have lost touch with for that reason and the others i personally don't want my issues dragging them down.he might just be pushing you away to protect you from their negative situation, I have done this so many times people.
But their is that one friend who i met in college with similar issues to me and even though we don't see each other physically that much we text and call each other at-least once a month because scheduling difficulties and the fact that i forget. Because i am forgetful my friend gets frustrated by accept that my brain is built differently.
Honestly ask them straight up if they actually want to do the project with you or is their something else you can do together instead would be a start. Is their anything that may be making him unreliable like any barriers such as health issues, transportation or other obligation such as work that may stop them from participating.
If he has done something that you disagree with,the only way you can deal with it is to stop being silent and talk to them about it because it will develop into something bigger or be brought up eventually at a later date.
Is this relationship just Platonic friends or is their feeling of more romantic because if its Platonic friend i can totally relate too.