Denis did not enlist and serve in WWI and he was too old in WWII. He does have WWI and WWII Draft Registration Cards but they were just that a civilian registration that did not mean a man was drafted. For WWI he filed Sept 1918 the war was over Nov 1918 and with his age and having a family was not drafted. For WWII the Registration was for men ages 42 to 64 but the Selective Service Lottery for conscription was for men up to age 42. In the 1930 census Dennis answered "NO" the question in column 30 "Whether a Veteran of U.S. Military or Naval Forces". So he didn't serve in WWI for sure.
As for sons: All WWII and later military records are not on the internet. The NARA has Army WWII Enlistments 1939 to 1946 but not any other services. The Army enlistment only gives year born and I have seen it be a year off depending on when, in relation to a birthday, one enlisted. They give a "nativity" county or state then place enlisted which maybe different than where lived. It does not give any other identifying information like parents or spouses or next of kin. So it is near impossible to find these sons of Denis and any possible military service from the internet alone. Unless you can prove those buried in a National Cemetery are the men you are looking for with the same names. So unless you have family that can tell you then your best bet is a death certificate as most if not all, after WWII, had a box asking if a Veteran.