He'll get used to it, puppies usually have to learn what wearing collars/harnesses/leashes is all about. Maybe do some sessions where you put it on him in the house and coax him around with some treats. Once he gets over the initial realization that wearing a harness does not actually paralyze him, he should be okay lol. But I wouldn't leave it on him unsupervised because it's pretty easy for dogs to chew them off.
Juno CGC: 3/31/09 "Reverse" Brindle, docked & floppy
Lucy: 2/1/06 Bichon/Poodle Mix
Happy: 1/2001, Rescued Border Collie Mix
RIP Molly, January 1997-April 8, 2009