Given:
At the same time of day, a man who is 6-feet tall casts a 4-foot long shadow
And his son casts a 3-foot long shadow
Let the height of the man's son = x
Using the ratio and proportions:
Height : Shadow
6 : 4
x : 3
So, using the cross multiplication:

So, the answer will be:
The height of the man's son = 4.5 foot