Please don't hit your kid-- he's at a developmental point of his age that standing up is when a lot of the fun happens/ laying down means the fun stops because he's expected to stop what he's doing and sleep. Those are likely his associations: stand up- autonomy, self-directed/ lay down: adult-led, no autonomy.
Your child is not a passive actor, and there's good in that. When the children I cared for started to get squirmy, we moved to 'standing up' diaper changes in the bathroom. (easy to clean if there's any accidental mess). This also worked great in preparing the child for having to change diapers in situations which laying them down was untenable (restrooms with no changing table-- it happens) and for potty training and changing pull-ups or underwear-- they'll be getting changed and cleaned up standing,not sitting.
It boggles the mind that we would treat this as a disobedience/control issue and further make it unpleasant in the hopes of teaching obedience when this is a child's cue to us that they are developing their own sense of self and priorities. I mean, think about it- if you had to do something you really didn't want to do, wouldn't you want some choice in the matter? I think most of us would answer yes.
So heed this warning-- if you go forward making this a power struggle at age 16 months old, be prepared for more problems. Because this is something very, very little in the big, big picture of parenting. Try the smallest changes first (standing up changes) before breaking out the big guns and making it a big, painful experience.