15 month old Masterbation

I am listening to your past calls about masterbation, but I'm not sure what to do for a 15 month old boy. He is a foster child. His case worker says it's normal baby behavior but I'm not sure. It feels excessive (several times a day and several times a night.) My little guy is a little developmentally delayed and we believe he has spend most of his 1st year in a crib or car seat. Only in the last month do I feel like he's starting to become aware enough to sort of understand what we are saying.

How do I help him stop this self soothing habit even though he is so little? And is this normal? Because I'm concerned this is a sign of previous sexual abuse (are Boys seem quite normal aside from some neglect) his older brother (3) hasn't show any signs of being sexually aware. Thoughts?