I am so sorry to hear that Marvin :( My condolences on your loss. Did you find him after he had already passed away?
Symptoms of heat stroke include:
Very heavy panting and respiratory distress
Vomiting and diarrhea
And Extremely high body temperature (often 105-108 degrees)
If he became worked up (barking, overly excited), in 90 degree weather- this can easily lead to heat stroke. Especially in a male "stud" dog if there are females around. Unfortunately, dogs don't always know when to take it easy, and they willingly go beyond the limits of what their bodies can take. In this type of weather, he could have easily pushed himself beyond what his body could handle. In the future, try to keep excitable dogs like him away from things that could stimulate him to get worked up, and it is best to keep in fully shaded areas when outdoors, or inside with air-conditioning.
If he had an underlying heart issue, or breathing problem such as elongated soft palate or laryngeal issues, this could also make him more prone to heat stroke.
Of course, there is the possibility it could have been another problem. He may have just had a faulty heart. Heart disease can be insidious and the heart can fail sometimes without any warning.
I hope my answers have provided some insight. Again, I am very sorry for your loss.