IMHO, your son is actually pretty lucky. Most boys don't learn this life lesson about "some" girls until much older. Some after their 4th or 5th divorce...
I know it is stressful, but such a blessing if used to teach him about healthy relationships. My S#2 had to learn it at 13, ouch... now that girl....
He has spidey senses, now. Fingers crossed it sticks.
Best frienemies.
It happens with all types of kid friendships, not just boy/girl.