If your Grandad married or had a child whilst he was in the army, you might find that his rank/regiment and service number are on the relevant certificate.
My grandad married in 1940 and his marriage cert gives the above info and then gives his 'normal' occupation underneath.
Using this info, I was able to send off for his service records from the army, which was fantastic. He was also a tank driver and was in the Royal Tank Regiment.
Have you got any photos of him where you might be able to recognise the uniform or cap badge or something?
As for the sweets company, have you any idea of the name of it? Depending on when it was, you could try looking in historical directories. Try here:
http://www.historicaldirectories.org/