I think you can let it defrost in the fridge and use. You can also heat with hot water (put it in a glass container in a bowl of hot water. I microwave mine but I know some people frown on this . Don't leave it out on the counter to defrost though. That's not safe.
I would not only say that in as far to kill bacteria etc, but If you know it has been refrigerated when cool and frozen within that day and then defrosted in the fridge, Id say you are pretty safe with anything not reheated. I would not stand rice to make room temp at all to be honest, Rice can be very dodgy. If its cooled quickly, refridgerated and frozen once cold then defrosted and eaten strait from the fridge should be ok, but I wouldnt allow to stand at room temp for any periods of time. Heat to boiling and allow to cool. I have done a food hygine training and made all my own babyfood and have never had any problems following these rules as you would for an adult, just being a little more religious about these rules with dd.
I wouldnt worry too much beatof, Im sure he will fine. The main problem is that children and infants can harbour stomach bugs bacteria that travel to thier bowels very quickly and can stay in intestine and bowels for upto 18mnths. but I would try to defrost in the fridge I sometimes would put in a cool bag if going out for the day etc.
just to add beat my now exp once (when I was away over night) left a bottle of formula sat on the radiatior all night so he didnt have to get up and make a bottle when dd woke up early in morning DD was absolutly fine though to my surprise.