My 4 yr old son bruised his tailbone. What is the best way to treat it and how long before he's 100%?

Leave it alone. It may take a while but mother nature will heal it. Just give reassurance.
Treat symptoms. A bruise on the tailbone is no different than a bruise anywhere else. Treat the discomfort and he will get better on his own. He will tell you when he is 100%, but depending on the severity may be a few weeks.