He was riding a flat tire. You say he was riding a lot this summer--did he pump the tires up very frequently? Tires lose pressure just from sitting with the weight of the bicycle on them, and they need to have their pressure checked on a regular basis. In addition, in summer, when temps are much higher, tire pressure is higher. If fall temps come in and you don't check your tire pressure, it's likely the tires have lost significant pressure. Whenever temperature drops significantly, a person needs to check the tire pressure.