The tissues in the nose can take a while to heal, which means once there's been one nosebleed, the chances of another happening within the next week or two are vastly higher. So nosebleeds will likely come in clusters.
What triggered the first nosebleed? Any one of a number of causes, including maybe just scratching at the nasal lining, or idly nosepicking (people do it, they just don't admit to it). You know the feeling - your nose feels a bit congested, you can h ear the air whistling in your nostrils, you know there's something there that won't come out with nose blowing, so you judiciously help it along discreetly. But kids are clumsy, they're more likely to do damage in the attempt. And knowing they shouldn't pick their nose, they will of coure insist they didn't.
Or maybe it was a sneeze, or maybe allergy/inflammation of the nasal lining. Or something more serious - you don't know. But I wouldn't worry too much at this stage unless it keeps on happening, and even after several weeks' break you get more clusters of nosebleeds.
Warm weather can be a major factor, or anything else that makes the blood vessels in the nose dilate and the outer walls to be a bit softer and more relaxed than usual. If rthere has been a recent nosebleed, the healing may be incomplete and break down too readily, with minimal provocation.
Just apply the usual first aid, teach him what to do and maybe help by applying ice as well, to help the blood vessels contract.
It's a nuisance when it keeps on happening.
Marg