Don't worry - you are close enough as you are a relative. But first you have to obtain his death Certificate £9 - you can get it by applying online. Then you will need to send this together with the application and £30 for his Service Record. The evidence of his death is the headstone photo and the entry on the CWGC database, so saving you at least £9 for a death certificate. As he died so long ago, you don't have to be a next of kin. Even I could apply for them.
I suspect he may have been wounded a day or so earlier and died of those wounds after being taken to a RAMC unit at Dunkirk.
