You can try running the humidifier, but if you have the windows open, it isn't going to do much good. You should try an enhanced nasal spray or gel. Neosporin used about three times a day will keep his nose moist too.

Do you think the spring allergies are just getting to him a lot?
I used to get bloody noses around this time due to pollen. It would explain a lot especially because you have been keeping the windows open. Pollen comes in.

You should make sure that she/he sits up and tilt there head down then hold it there for 10 min after that make sure the kid does not run around for about an hour. Dont worry if it last for a while but if it lasts more then 30 min you should call your doctor and ask for what to do.