The purpose of basal/long acting insulin such as Levemir is to keep our levels stable in the absence of food, exercise, fast acting insulin, etc., such as overnight.
So whether a morning BG level in the 4s is ok depends what his level was when he went to bed. If he went to bed with a BG in the 4s, Levemir is doing it’s job. If he went to bed with a BG in the 8s or 9s, his levels are dropping too far and his Levemir dose is likely to be too high.
That said, it is often recommended to go to bed with levels on the higher side to provide a bit of leeway in case they drop overnight. It is not usually recommended to go to bed with levels in the 4s. Often, levels around 8 are suggested.
The profile for short acting insulin such as NovoRapid shows that typically (we are all slightly different) it remains active for about 4 hours. After this, it has no (or very little) affect on BG levels. So a NovoRapid dose for breakfast is unlikely to have an affect at lunchtime and beyond. If you find your son’s levels are dropping during the day, this is further evidence the Levemir dose is too high as it will be the only active insulin at that time.
As
@Rokaab says, the forum does not allow members to advise on medication doses as we are not doctors. So, my advice is to talk to your son’s nurse about how and when to reduce his insulin dose. It is useful to learn the basics as doses rarely remain stable for long so we need to learn about adjusting doses. I appreciate it is still early days for you so don’t expect to know everything.