At 5 1/2 months he should be almost done losing his baby teeth, so I bet he has been losing them for awhile and you just haven't noticed.
Usually the very final baby teeth to go are the canines at about 6 months give or take a bit.
As far as the health and safety of rawhide, that is a very personal decision. I do give my dogs rawhide on occasion, but generally only retriever rolls that are made from one entire piece -- never ones that are pieced together in the factory like so many of them are.
I generally give nylabones, and antlers.