Dear Ms. Brescia,
Thank you for posting your request on History Hub!
It may be possible to research your relative's service if you have some basic information on where he might have resided within the United States, his date of birth, or general age, information on relatives he may have listed as next of kin, and what conflict he may have served in. From there, you could conduct a search of WW1 and WW2 draft registration cards, as a starting point for your research. All WW1 and WW2 draft cards are digitized online and are available at both Ancestry.com (a paid service, but many local libraries have a subscription you can use at their branches) or through FamilySearch.org, which is a free service.
Here is a link to the catalog entry on FamilySearch for their digitized records.
We hope this is helpful. Best of luck with your research!