Ewan, if your Son's scope has some lines or dots then it's perfectly fine to range-find using the bracketing method.
In short, the gap between the target hinge and the centre of the kill is the same distance regardless of wether it's a pigeon, rat or rabbit.
Some people try and use the kill to bracket, this is a mistake as it is too small, hinge to centre of kill is bigger so less prone to error.
This allows a shooter to use this constant to 'bracket' using lines in the scope and work out the distance.
All you need to do is get hold of a standard target and tape, put the target out at the usual distances i.e. 30-55yds and try and find a zoom setting that allows you to bracket.
Sounds complicated but in practice once you know that say from the centre of the reticle to three mil-dots down is 50yds then it's easy to see that 2.5mildots will be 55yds, 4 mildots will be 45yds etc.