kimifin, I don't think it is advisable to add another dose of the medication, have to follow the label closely. Btw, I was told the correct way to take medication. For example,running nose med, if the label says 3 time daily. I will give 8 hours apart. However, it will be difficult to give at night. So the last dose I will shorten the interval to maybe 6 hours apart.
Like you, I was so tempted to give extra dose when seeing my kid so uncomfortable. However, I have never done so, in case of overdose. Especially they are still so young.
You can try to massage your boy's limps, body, face so that it will help to relax him. Easier to make him sleep again.
Hope he recovers soon.