Let Zainal's age be x; Son's be y.
(This year)
Zainal's age: x = 4y
(Six years later)
Zainal's age: 4y + 6
Son's age: y + 6
The sum of their age: 52
Hence (4y + 6) + (y + 6) = 52
5y + 12 = 52
5y = 40
y = 8.
In conclusion, the present age of his son is 8 years old.