My Grandfather (Stan Cancea) came over from Romania in 1908 and we believe was made a British citizen after the 2nd World War. Unfortunately the National Archives cannot find any record, apparently some papers were lost or damaged. My Grandma received her naturalisation in 1943 (having to become a Romanian to marry) but we have hit a dead end.
The London Gazette would normally print a notice. Here's an example page showing naturalisations in May 1939: http://thegazette.co.uk/London/issue/34634/page/3881 (my grandfather, who came over from Germany, is featured).
I can't locate your grandfather by searching, but I suspect he's there somewhere. His name could be mis-spelled (or not properly indexed) - it might be worth trying some variations on this search.
[unfortunately the Gazette site search doesn't seem to allow wildcards but you might have some luck doing a Google search on with site:thegazette.co.uk, as the individual pages do seem to be indexed]