4yr old Vyvanse | ADHD Information

My son was just diagnosed with ADHD on Friday Dec 10th
and we have started Vyvanse for him. The child psychiatrist said to try half of a 20mg capsule in the
mornings and if nothing changes after 2 weeks to up the
dose to a full 20mg capsule. So far we have only been on
it 3 days as of today and I have noticed a slight
difference the first day but then yesterday he slept most
of the day and was irritable.

My question is how do I know I am only giving him half of
the capsule and could the different reactions on the
first 2 days be because of giving more medication on the
second day? I am dissolving half of the capsule in water
for him every morning if that makes any difference.