When I find myself intensely falling for someone and my brain starts acting funny like this, one way I try to temper my feelings is to remind myself that the guy isn’t perfect and I focus on flaws they have as a way to bring myself back down to earth.
If this guy does end up breaking up with his girlfriend, I’d caution you to not immediately jump into a relationship with him. Give him time to grieve for the relationship. Be his friend, not his rebound!