I cannot be sure of him. There is only one William Born in Belfast in 1900, but that was 6 Sep in GRO, but the same man joins RAF with 10 Jul 1900 given as DOB. This man told RIC Apr 19 1900, and therfe is nothing to match that
1900 Apr 19 born Belfast
1921 Jun 9 enlisted with RIC no 81424 Boilermaker & ex-soldier