Hi jo,
I too find that my kids sometimes don't recognise that they are hypo when they have a reading in the 2's, and other times they may be around 4 and tell me they feel low. My husband who is also diabetic says that the symptoms of a hypo can vary and sometimes for him it can be an emotional feeling. As your sons is newly diagnosed, it may take him sometime to notice the way his body feels, but often with kids they are so engaged with what they are doing that they don't realise the signs.
At school I normally send mine in with a small fruit snack which they have around 10am ( 2 hours after breakfast). We find that having a small amount of fruit 5 - 10g of carb can help to prevent them dropping too low before lunch. They also have a small amount of carb before a PE lesson.
If you are concerned about him dropping low at school it might be worth you asking the school to do a mid morning blood check for a few weeks so you can see what's happening at this time. Hope this helps. Leggott