I don’t think that there’s anything particularly unusual in a memoir in omitting a visit to their son, who may have asked not to be included — he seems to only feature in TSP in a single conversation, in my memory?
There are two dodgy bits to it, in my view. One is that Tim was well enough to be learning to surf at a time when he was receiving a terminal diagnosis of a serious neurological disorder. The other is that they clearly got on well with their son, and any sane couple in that situation would have said something like "Son, we're about to be made homeless and we won't have any money. Could we kip on your floor/borrow a couple of hundred quid?" I mean, you just would, wouldn't you? Even if you were reluctant to impose on your children, you would still do that.
And then there's the business about meeting the son in the middle of the holiday. His post reads "'3.5 hours to Bristol 30 mins break then floor it back fun times . . . dropped my parents off heading back now", that suggests they met in Bristol, so how did they get there? Did their son pick them up in St Ives and drive them to Bristol and then drive them back again? That doesn't make a whole lot of sense if he was living in Newquay and they were in St Ives.