You can ask for a Mandatory Reconsideration - as Tiger Rag said he has 28 days from the date of the letter stating he didn't get it. If that fails (most MRs do), he can appeal to HMCTS (Her Majesty's Courts and Tribunal Service) for a first tier tribunal. They are independent from the DWP and have a much higher success rate than MRs do. I would suggest getting help from CAB or another charity to make sure you get the MR letter written correctly. I would also suggest that phoning the DWP to tell them you want to do an MR is a good idea, and say you're going to be writing a letter as well.
Also, he could potentially apply for Employment and Support Allowance as he is unable to work due to his health.