When DD was younger she got a fever during the day and was quite unwell. Then later that night her temp dropped to 35 something. She also got a rash. So we called the nurse number and they said that anything below 36 is considered a low temperature. I'm not sure if it's different for older kids, but I'd be keeping an eye on him, if his temp is low I don't think you'd want him getting lethargic.