My brother and my nephew were very short children. I don't think there is much to do except maybe buy shorter clothes for them. I am puzzled by your question what can his parents do? I presume he saw HVs and probably doctors along the way who were not concerned.
Yes you should be concerned and the GP should be looking at potential GI issues. A mother I exercise with is going through this with her son. Turned out he has celiac disease as well as a host of other allergies. They have changed his diet plus he is on medication and he is now growing.
Are you sure his height is correct and that you're reading the chart right? DD has always been around the 9th centile, sometimes a little below, and is absolutely fine - a healthy little competitive gymnast
But if he is really below the 1st centile I think a visit to the Drs would be in order. Below the 2nd centile is when they like to double check stuff I think.
But do be sure you have his height correct - a few cms can make a lot of difference at the bottom end between 'small but okay' and 'needs checking out'.
This is quite a useful set of guidelines that go with the U.K growth charts (are you in Aus if using Aussie ones?)
They say no measurement indicates a problem by itself, but only 1 in 250 kids will be below the 0.4th centile, so these children should be assessed further.
It also shows you how to do the mid parental centile, to work out (very) roughly where you'd expect him to be based on parental heights. That has him at 25th-50th centile so again a discrepancy.
I think his parents have to make the decision to get him checked though. If you want to gently suggest anything I would think a trip to the GP or health visitor to get him properly measured would be Step 1.
Children with growth hormone deficiencies, for example, classically present at around this age. Earlier their growth is driven more by other factors, then starts to tail off. Depending on how close you are, I would just gently ask if his growth has been looked at recently. They may say it is in process - it isn't a rush to do something just as long as it is being looked into. Depending on actual measurements a period of watching with accurate measurements could be helpful. If they haven't taken him to the GP about it then encourage them to do so, because yes it does sound like he's at a stage where some basic investigations are appropriate.