Does your SIL pay his own Income Tax and National Insurance or are those deducted by his employer? Does he get paid holidays?
If he's on PAYE and getting paid holidays then he's definitely an employee of the firm (rather than self-employed). If those things don't apply then he's being regarded as self-employed but, if he's not free to accept or reject work at will, HMRC will probably regard him as an employee (meaning that he should still receive the rights afforded to an employee).
If your SIL is an employee he should have been provided with a 'written statement of employment particulars' within two months of starting work (or, if he was originally self-employed, within two months of becoming an employee of the business):
https://www.gov.uk/employment-contracts-and-conditions/written-statement-of-employment-particulars
A 'written statement of employment particulars' isn't exactly the same as a contract (which doesn't have to be provided in writing) but it encompasses many elements of it.
If your SIL is 'laid off' during inclement weather then he should receive full pay unless his contract (or written statement of employment particulars) provides otherwise. Even if the relevant document states that he won't be paid he's still entitled to £28 per day, for a maximum of 5 days in any 3 month period, as 'guarantee pay':
https://www.gov.uk/lay-offs-short-timeworking
For further advice (and support in getting things changed if he's genuinely an employee) your SIL should call the ACAS Helpline on 0300 123 1100 (M-F, 0800-1800). Several AB members have reorted quite long waits to get through but have said that the wait was worth it.
The foregoing assumes that your SIL isn't on a 'zero hours contract' (as, say, over 90% of workers in Wetherspoon's pubs are). That would severely reduce his rights (possibly to none) in the event of work being curtailed by poor weather. However employees on zero hours contracts would normally have a written contract stating so (which doesn't seem to apply in your SIL's case).