I used to put the water ready in sterilised bottles and put the powder in a little pot ready for nursery so they just had to add it in and warm it (or give it at room tepm if he will take it that way!) Or the other thing you can do is put the water ready the night before and refridgerate it then add the powder before you leave and put it in a cool bag so it is chilled but hasn't been made up for too long.
It depends what you are comfortable with, lots of people make up feeds for the day, we keep bottles of water ready in the fridge. In the UK they say to make feeds fresh with freshly bolied water around 70 degrees bacause the powder isn't sterile and that should kill any bugs - but we have never had any problems adding powder to cold water and heating!
Laura