Translation:He wears underwear.
I checked with a friend recently - she said pantsu has been used to mean underpants but currently people use both pantsu and zubon for pants/slacks (also I'm guessing that pantsu is prob also used for underpants still, the meaning is evolving so that now it is used for either). She didn't say anything about shitagi. I don't think I asked about that and my guess is it's probably not used all that often now - not sure what the modern word for underpants might be.
Japanese teacher here... I have always used パンツ to mean WOMEN'S underwear, the lacy kind. It's from the word "panties."
However, young whippersnappers nowadays are using it to mean "slacks/ pants." But I would never ever imagine パンツ to be tidy whities or boxers. I would use したぎ（下着） (literally: underclothes) for that.
I've read the comments about underwear vs trousers vs pants vs zubon (trousers) vs shitagi (underwear). I've tried "slacks" (that's a no). I haven't tried "panties." I translated this sentence to a simple, straightforward "He wears trousers." DL marked it incorrect and indicated it means "He wears underwear." Okay. I hope I never encounter this sentence again because I don't see the point of discussing underwear in general conversation unless it's bullet-proof or has magical properties. On a positive note, I did learn the difference between kimasu and hakimasu from several comments. Thank you for that.