My kid taking up with the class "bad kid"( kinda long, sorry)

Hi all. I haven't posted in a while as things have been busy but good.

Here's the dilemma. My son is in kindergarten. There is a boy who insists on
hanging around with my son all the time. This kid is really bad news. He is
constantly at the principles office for swearing, hitting and stealing the
toys. My son, who thinks that everyone has HIS best interest in mind at all
times, doesn't recognize this as really terrible behavior for a 5 yr. old.
He doesn't do any of these things at home and so far hasn't started
following example at school.
None of the other kids go around him because they know this kid is a bad
seed.
I talked to his teacher about it and she told me that my son was saving
spots for him and playing with him allot. My son says he's scared of this
kid and doesn't want to hurt his feelings because none of the other kids
will play with him. The teacher said she would try to separate them. Maybe
I'm over protective but he hasn't had the chance to make other friends yet
and this little brat is already all over him.
What can I tell him at home to help him see that this is not a healthy
friendship or a good situation to be in and that his should avoid this kid
at all cost?

thanks for any advice!

s