3.4
As you need it for medicine therefore, I will try to give you more precise value:
Usually, 1 ml = 0.202884136 teaspoons
And 1 teaspoon = 4.95 ml
And you need to give your son 2/3 teaspoon which is equal to = 0.7 teaspoon
Therefore, for converting 0.7 teaspoon into ml:
0.7 teaspoon = 4.95*0.7 = 3.465 ml
Usually, 1 ml = 0.202884136 teaspoons
And 1 teaspoon = 4.95 ml
And you need to give your son 2/3 teaspoon which is equal to = 0.7 teaspoon
Therefore, for converting 0.7 teaspoon into ml:
0.7 teaspoon = 4.95*0.7 = 3.465 ml