Your son will need to read the specific provisions of the particular policy that he took out in order to obtain a definitive answer. However, looking at this policy from Aviva as an example
https://www.aviva.co.uk/content/dam/aviva-public/gb/pdfs/personal/insurance/travel/policy_document_insurance_travel_ntrtg10145_pre.pdf
I doubt that he's covered for such problems.
The section headed "Missed connection outside of the UK" (page 15) only covers delays or cancellations to scheduled public service transport. There's nothing there about delays due to the actions of officials of government agencies. As the "General exclusions and conditions" section (page 9) states that 'any loss that is not specifically described in the cover sections of this policy' is excluded from cover, I can't see any way that a claim for delays caused by US Border Agency employees would be valid under such a policy.
Sorry!