Where do you get the information from that he can apply simultaneously?He can apply simultaneously and the two applications will run concurrently but this should be flagged in both applications by means of covering letters. Assuming all requirements are met the PR will in effect be "backdated" therefore the need for the 1 year post-PR wait before applying for naturalisation will be satisfied.
Of course he doesn't need to wait one year after receiving the DCPR if he supplied evidence with the application that ended at least one year ago. But there still remains the requirement to send in the DCPR with the AN application - I wouldn't rely on the two UKVI/HO teams to work together just because covering letters are supplied, if you get a 'nasty' caseworker you may get a refusal. Rather wait 8 weeks for the DCPR...