Finding a Walt
Tell him you are collecting the service numbers of ex service friends for comparison. If he doesn't spout it very fast he didn't have one.
If you meet his wife ask her, she will too. Service number was essential knowledge for every wife for all sorts of issues like medical treatment, station library, station nursery school etc.
There is a large service manual called "the retired officers list", we used to ink out the re, you could check his name in it or get a service chum to do so. There is one on most units so any serving person can help. You might be able to get access at one of the stationary office stores or even ebay