Thanks for your reply, Gibel
I have found no actual parish register for his birth, only his marriage and death. The information may have come from others who are also researching him and while it may not be entirely reliable, it seems feasible, looking at the other information on the family tree, that the date is not too ridiculous to be discounted.
Since my post, I've checked on various sites and members have suggested that the Freestones/Fristons of Cambridge came from Suffolk. Again, I don't have the evidence to support this, but it is a lead that may be worth pursuing.
There is a record of a Thomas Friston son of Christopher and Mary born Aug 19th 1718 in Withersfield, Suffolk.