Secrets of Droon (special editions are more intense and you may want to skip them)
Flat Stanley
Mrs. Piggle-Wiggle
Princess Tales by Gail Carson Levine (despite name they're not necessarily super-girly)
Regarding the . . . series by Kate Klise (if he can read cursive)

You might find some ideas on this Good reads list of similar books, but I don't know anything about them directly.
https://www.goodreads.com/book/similar/1484561-how-to-train-your-dragon

My DD who also liked that series also loved in the fantasy genre:

Land of Stories/Tale of Magic
Wings of Fire
Fairy Tale Reform School, starts with "Flunked"

If what appeals to your kid is the "absurdism" in HT Train Your Dragon, we have liked Wayside Stories and also Mysterious Benedict Society, which have that similar off-beat sense of humor.

If what appeals to your kid is the adventure / fantasy aspect, you could try the Tui Sutherland dragon books (Wings of Fire.) I found these a little sing-songy after about three of them, but they do present an alternative dragon world...

My 2nd grade DS is LOVING the How to Train Your Dragon Series. He has 2 left. What are other series that are similar? Note: he does not like anything with death or blood. He also loved her next series Wizards of Once.

Thanks!