From Ancestry explaining the WWII Draft Cards and registration database on website. "This database is an indexed collection of the draft cards from the Fourth Registration, the only registration currently available to the public (the other registrations are not available due to privacy laws). The Fourth Registration, often referred to as the "old man's registration", was conducted on 27 April 1942 and registered men who born on or between 28 April 1877 and 16 February 1897 - men who were between 45 and 64 years old - and who were not already in the military."
So those born after Feb 17, 1897, if they registered for the draft are not available at this time.
There is an an US Army WWII Enlistment database at Ancestry and the NARA website. The database does not cite whwter they were drafted/conscripted or volunteered. The Navy/Marine on isn't online yet.
You may find th sons marrried in 1930 US Federal census.
By the way Denis might have a WWI Draft Registration Card but bd is different from what you have and this is WII forum. Same with WWII but again different birthdate of yours and WWI card. Both have first name as Dennis.
See if your libray has a subscription genealogy program for you to use.