Sue, I am a hobbyist only, not an expert. I will share my thoughts, but confirm all independently.
I don't think the dog tags are WWII vintage. It appears to me the "55541518" is his serial number. No WWII serial numbers began with a five. The "US" is one of several prefixes added to army dog tags in 1949. It signifies "Enlisted Soldier Army of the United States".
The "T56" would indicate his last tetanus shot was in 1956.
The "O" would indicate type O blood.
I wasn't able to find anything that would shed light on "NASAR3H5 or N5". Maybe someone else can help with that part.
Here are two web pages with some or all of the above info: