This is happening to a lot of people and is unfair but not unexpected given how the economy has been burned out. If your son had a signed contract with his new employer he will be entitled to pay in lieu of contractual notice, but if his contract only allowed for one weeks notice during a probation period (which is common) for a new starter, it won't be much money, and no one who has worked less than two years with an employer qualifies for any redundancy pay in UK , so even if he had started he would not have received anything by way of compensation. He may qualify for job seekers allowance of circa £75.00 a week which is not means tested, if he has paid enough NI contributions, but this is potentially taxable if his income in the year it is paid exceeds personal tax allowance. Universal credit is only available if he doesn't have a partner who works. If he is a single person with no dependents, and has less than £16k savings, he may be able to apply but it is unlikely not be enough to pay rent/mortgage as it is only about £400 per month for a single adult living away from home.
My friends who are a couple who are both furloughed are currently v receiving £5000k a month. However even they are at risk of redundancy but unless they both get made redundant, ( which would be dreadful) they will not get universal credit. It is absurd and unfair, but furlough pay level was not means tested as it was a benefit to the employer to stave off mass redundancies temporarily. No previous employer was realistically ever going to re-employ ex employees and then pick up the cost of possible redundancy payments. The Government should not have publicised this an optionvas it was only ever theoretical in practice for most people who changed jobs in February/March.
The so called safety net only really applies to families, not single adults, and is not available if you have saved a little over 16k, which might be the case if saving for deposit on a home.
The only people I know who have not been impacted financially are
either key workers still working, or existing pensioners with guaranteed pensions in payment and with mortgages paid off. Even people about to retire have seen the value of their pension pots crash in value.
Hopefully your son will find a new job soon. Please don't let him become disheartened, and even if you can't help him financially yourself, do try to support in whatever way you can as impact on his mental health may be considerable.