The age of a father today is thrice as that of his son. After $15$ years, the age of the father will be twice that of his son. The age of the son today is:

Let the present age of the son be $S$ years.
Consequently, the present age of the father is $F=3S$ years.

After $15$ years:

* Son's age will be $S+15$
* Father's age will be $F+15=3S+15$

According to the given condition:

$$3S+15=2(S+15)$$

$$3S+15=2S+30$$

$$3S-2S=30-15$$

$$S=15$$

Therefore, the present age of the son is $15$ years.