12 week old puppy loses his mind if other dogs get too close

I don’t know if he’s already become reactive or if it’s normal. He’s a 12 week old GSD. His dog experiences are the following: 1. Last weekend a playdate with friend’s large senior dog. He barked at her for two minutes or so on and off at the beginning while sniffing her before engaging in playing with her. Had a great time. 2. Weekly puppy class. There’s no free play but we are nearby 4-5 other puppies for an hour. He will bark at them at times on and off during the class, mainly if they lunge towards him for some reason or we get too close (like within 5 feet). But he’s easily pulled away and then focuses back on me and will settle for long periods during the class.

Today we did some intentional dog exposure where I took him to a high traffic dog area. We sat in my trunk and watched people with their dogs. If a dog was 20-30 feet away, he watched calmly/intently. HOWEVER if any dog got within 10 feet of our car, he would lose his mind. Barking like crazy until the dog left that radius. I gave him lots of treats during this session (like 15 minutes total).

Is he reactive already? Is he under socialized? I’m really scared of having him be unable to tolerate dogs and I just don’t know if this is normal puppy behavior or a deeper issue already.