OK.... this is the way i look at it.... having spoken in detail to my uncle who developes / makes / invents drugs and medicines etc....
In order for a medicine to even get a license a slight (or even quite major) error in the way / amount given must not cause seriouse side effects.... even more so for over the counter non prescribed medicines etc...
Medicine is also specific mainly to weight (which is why if you are in hospital etc the like to know your weight) and looking at it another way, at vets they weigh the pet and give X amount per KG etc
They just do it by age as thats an "easy" way of describing how to do it!
Therefor in order for medised to even have its license for a 3 month old baby it would have to be safe for a baby on about the 1st percentile (or smaller!) to recieve an overdose without seriouse consequences
Given that you baby is just a few weeks away from this milestone i would imagine it would be VERY unlikely to do harm (or else it would not be allowed to be on sale in the 1st place!)
Most of the guidelines etc are to cover their own backs etc....
so as long as the child was of the size as a small 3 month old, i personally would give some medised to him (the 3 month one obviously, there are 2 versions , one from 3 months, the other from 2 years)
I would however only do thise for MY OWN child before any one questions me doing this as a nanny!!!!!!!