Heatstroke or something else?

I had a 14 week old cockerel Buckeye die this morning. He weighed 3 lbs 10 oz, the same as he weighed 2 weeks ago. When I let them out, he wanted to lay down in the feed shed. I scooted him out and saw he was walking very knock kneed. It was a very different gait than normal and he kept laying down. His comb was also very pale pink and he was visibly struggling to breathe. It was at least 86 degrees yesterday but extremely humid, down to 72 last night and cool yet this morning. He drank some electrolytes when I separated him.

While I was handling him, he started to seize. And then continued to jerk for a few seconds and stopped breathing.
He had some white on the end of his feathers around his vent. He smelled kind of bad. There was also some fresh foamy watery white poop and another spot of light yellow poop near the shed. Didn't see who pooped it though.

Does that sound like anything? Heatstroke even though it was cool this morning?

Same feed we've been using. They free range most of the day.

Other random- I did have a chicken get accidentally ran over yesterday while we were playing ball with my puppy. It was a total accident, not aggression. I couldn't see the number on the chicken to know which one it even was. Chicken was flattened but got up and ran away.

I'm worried about the rest of the flock and have electrolytes out as well as cool water.