My dog stiffens in his sweater. How can I get him used to it?

Normally Havanese have enough hair not to be cold
and would not need a sweater. A sweater may be itchy and restricting.
If you really think Kiba needs to wear a sweater, make sure it is not too tight, and put it on often, so he gets used to it, but do not leave it on for long. It is also important that he is also well brushed and has no mats. Combine the wearing of the sweater with something he likes to do, ie going for a walk or starting to play with a ball. Make putting a sweater on a ritual before the fun activity so he makes a connection.