I agree with Sarah’s analysis.
If the client is against having a substitution clause, he needs to recognise that if both children die before him the entire estate passes to his grandchildren. Should he be against that proposition, then he needs to do something about it. However, if he is content to accept that situation, he also needs to be made aware that view is based upon the law as it currently stands, and there can be no certainty that the law will remain unaltered.
Should his son’s circumstances change, of course, that could also torpedo the client’s expectations.
Looks to me that the will file may need to include letters to the client clearly setting out the options that he will need to consider, whether in support of telephone/attendance notes or as stand-alone advice.