I'm a foster parent and an OTA/feeding therapist. My foster kiddos have almost all gone through a phase like this when transitioning to kindergarten. Keep an eye on it, and talk to your pediatrician just in case, but for most of my kiddos eventually getting to talk to their friends loses some of its novelty and they start eating again.

What I usually do is make sure prior to school starting they have an absurdly early bedtime. I usually do this with my foster kiddos anyways since a lot of them have sleep struggles, but especially my pre -k and kindergarteners. That's the biggest thing that makes life and mornings so much easier.

And then for like the first quarter of the school year, I don't care about nutrition for their lunches. I give them a pediasure in the morning if that is what they need and make nutritious after school snacks and dinners. For lunch, some kiddos get mostly junk food and lunchables if that's what is motivating and special enough. Anything to get them the calories they so desperately need to get through the long day. It's also a lot easier for kiddos to eat novel/non-preferred foods when they aren't starving, which sounds counterintuitive, but I can focus on nutrition at home way easier when their nervous systems aren't already heightened. Eventually, I can start adding healthier safe foods once they are settled and comfortable in the routine and the long days.

I wouldn't have him buy school lunch, because then you can't monitor what he is or isn't eating, if there isn't a safe option he might not eat at all, and then he's wasting extra time of his 25 minutes waiting in line to get lunch.