Hi Polly
Welcome to the forum.
You might want to have a look at the following FAQ which explains where everything is;
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum As your son has supplied a fit note, he is now in the assessment phase, the assessment phase is for 13 weeks, what this means is when your son is assessed and he is successful and placed in the LCWRA group then this will be backdated to the 14 week his assessment phase.
It can take many weeks from handing in a fit note to receiving a Work Capability Assessment form, check out our guides on how to fill out the form;
help-for-claimants/esa1 . Your son will need to keep supplying fit notes until his workcoach informs him that he no longer needs to send in any more.
We do have members on the forum waiting over a year to get an assessment, some members report that they have been quite quick, so it depends on where you live in the region as to the time frame.
Gary