If your son was asked a question about modifications and he said there weren't any, then his insurers will have voided the policy due to misrepresentation and non-disclosure (technically different defences but are generally always raised at the same time, i.e, he misrepresented the fact that there were no modifications and didn't disclose the fact that were modifications).
Should this get legal, the insurers will apply the 'inducement test' per Pan Atlantic v Pine Top (1995) as far as materiality is concerned i,e were the insurers induced into entering the contract because thery were told there were no modifications or were not told there were modifications, and had they known they would not have entered in the contract.
Your son would need to demonstrate that even if the insurer had known about the modifications, the insurer would still have accepted the risk. However, where the court decides an item is a common material fact that insurers would want to know about (and modifications certainly are), then the contention that the insurer would have accepted the risk anyway is likely to fail.
This all goes back to the doctrine of utmost good faith. Your son knows more about his circumstances and car than the insurer does, and therefore your son has a positive duty to disclose everything that is likely to influence an underwriter's acceptance of a risk.
If they asked the question and your son lied, then there really is nothing he can do.
If they didn't ask the question, then your son must challenge the decision.