He would have been 18 in 1945. He easily could have lied about his age and joined in 1943.
I know because my Uncle lied about his age and joined in 1942 at age 15.
I think quite a few lied about their age to go to WWII. My father-in-law was in combat in Europe on his 17th birthday, so with training- even if it was short training he had to be 15-16 when he enlisted. I don’t think it was considered a big deal then, they just needed so many and most considered teen age boys to be men then anyway since most did a man’s day of work.