Don't worry.
15 months is also a growth-spurt period and other developmental changes occurs.
My son... moves a lot when sleeping.
My daughter.... talks in her sleep.
No kid, or adult, sleeps still as a statue, nor all night.
Does he sleep late at night?
What is his bedtime?
AND, bear in mind, that a kid this age... starts to have, DREAMS too, or night-mares. Or Night Terrors.
At this age, it is their cognition that is changing too.
And cognition, is not just something that happens during the daytime.
My kids at that age, had Night Terrors and dreaming. And they grew a ton and hit milestones at that age.
And yes, it can be teething too.