It is ridiculously difficult when your income is variable, especially if you're not on PAYE, though I have always found the people at SFE are really helpful if you phone them. Don't let your DC out of your clutches until you know his password and student number and have got him to tell SFE that you have permission to speak on his behalf!
Fonclette's right, you can complete a CYI form when you are certain that your income will decrease but there is a possibility, depending on your timescale, that your DC will start the next academic year with a loan assessed on the income figures you submit for the 2013/14 tax year.
If his loan is reassessed on CYI during the academic year then he will receive the money backdated but you need to be prepared to subsidise the shortfall until that comes through.
However, if your income for the current year is greater than you estimated then any grant overpayment will be clawed back. You will also be asked to complete another form at intervals during the current year so the loan/grant can go up or down each term. It makes budgeting tricky, to say the least!
Despite these hassles, it is definitely worth doing a CYI application if your income drops so significantly. If your income for the entire year does turn out to be as bad as you fear then the grant element that he receives is not part of his student debt and there's a strong chance that he would be eligible for a bursary from the university. You should be able to find out on the uni website.
SFE should inform the university of eligibility for bursary but you should check on this. The process might vary between institutions, but there should be no need to re-submit income figures to the uni. Again, be prepared for delay.
Good luck looking for a new job and I hope it all turns out well.
PS. Keep copies of all forms you submit.