Couple things..
1 - I am impressed he was able to sleep at 4.2 lol Usually i wake up and have trouble sleeping if my sugars get below 5 lol
2 - The headache may be because he was so used to high sugars that 5, is relatively low, so his body feels 'low' when its still normalizing.
3- You should still be careful about those readings because glucometers have an error range. So it could SAY 4.2 and actually be 3.8, or 5... and then when you checked again and it SAID 5 and you thought 'its gone up' well maybe the first one was error low, and this is error high and really nothings actually changed either way. So it didn't 'go up' at all.
I learned this because i checked my sugars, got 6.2 on my freestyle and then got busy and 5 minutes later checked them again and got 6.9. That was 5 min apart, without eating or drinking or anything.. So its possible that a 5 isn't a 5, or a 4.2 isn't a 4.2 the readings could be higher or lower.
Also, teenagers can have attitudes lol Especially when all they want to do is sleep and mum keeps waking them and hassling them about stuff they don't care about, but thats your job, so if that way my kid i wouldn't worry about it hell thank you when hes older lol So hang in there until then! haha