If you've found nothing, then you might consider that, as it's at the seaside, there is a good possibility that he was a sailor/arrived by boat and was docking/trading at the local port (Porth/St Columb Minor).
So, realistically, if you can't find him anywhere where you're looking, it might be because he's from somewhere else!
Just a thought. At the time you're speaking of, getting around by boat was more common than using roads too. There'd have been locals going along the coast for pleasure/trade - as well as far-flung sailors arriving/departing and trading.
However....
FreeREG tells me that he's from "St Austle" - which is St Austell, Cornwall. On the other side of the county (10 miles in a straight line).
FreeREG has two that might be him being baptised in St Austell:
10 Jun 1751. Parents Lewis & Grace Roberts
26 Aug 1765. Parents William & Elizabeth Roberts.
Both at Holy Trinity.
