As I understand things, but have no experience of CIS
If self employed you can claim for any and all legitimate so called business expenses, such as.
Cost of mobile phone, purchase, calls, Etc.
Travel cost, which will include cost of any sort of travel associated with work, such as HP for the vehicle, fuel, and associated mileage, cost of maintenance and Garage bills [assuming he has some sort of vehicle] even a motor bike?
Cleaning of work clothing.
Cost of landline, his boss may need to call him on that phone.
Cost of clothing including PPE and safety gear, boots Etc. keep receipts!
If he does any work from home, such as paperwork then such activity can attract some tax deductions, cost of light, heat Etc.
Now for a potential kicker??
If your relative is only working for one person [firm] then he is not at times in the eyes of the Revenue actually "Self Employed" to be "Self Employed" a person has to work for more than one "Employer"
Recent [Pimlicoe Plumbers] court case may mean that the Claimant won the right to holiday pay Etc, Etc, Etc
OK that is now Law, but that does not mean that it will work in practice.
I am with MITCH on this one see if your Relative can find shall I call it more "Formal" work as an Apprentice.
Ken.