Just a few days back my 16 month old toddler started randomly spitting out (not swallowing) milk that he just put in his mouth. He kind of just lets it dribble down out of his mouth, almost like he doesn't care. I'm not sure if this is a real issue or if he is just experimenting with his mouth. Any thoughts?
It's almost certainly just experimentation. My son did the same thing at similar age, experimenting with letting drinks or chewed food fall out of his slack mouth, or holding milk in and swishing it around a while before either swallowing or dribbling it out. He would either act as if he didn't notice it was happening, or take a sneaky look at me to see my reaction!
If it seems like he actually -can't- swallow, then you should consult a doctor. If you have no other concerns about his swallowing or feeding (he takes solids well, he doesn't have unusual degree of choking or reflux, his doctor is satisfied with his growth and weight, etc), I think it's best to ignore it and the stage should pass pretty soon.