C.G.
I like the responses of talking to the other kid and his teacher, since both teachers need to be aware of the situation, as well as the bus drivers, and the kids need to know that it's not o.k. to bully someone. However, this could also make things worse for your son, as the bully may find other situations in which to cause him grief, using "mommy saved you" as an additional insult.
Does your son have any idea why things changed between him and this boy? Getting to the root of the problem seems most important. At the school I work at, there are "peer mediators", who are older kids (5th grade) that are trained by the guidance counselor to mediate between kids with these kinds of issues. It is very effective in helping kids deal with these problems, since the peer mediators are trained not to be judgemental, but rather draw out the underlying issues and help the kids come up with an acceptable solution. Also, since it is between kids, they are more likely to open up. The peer mediators also warn both kids at the beginning of the session that they must report to adults certain types of behaviors or problems, but the actual session is without adults, so kids are more likely to be honest. It has proved very effective. Why don't you find out if such a program exists in your school? If it does, referral to a mediated session could be requested by your son, or by his teacher.
Another more immediate solution would be talking with the bus driver to see if there could be assigned seats on the bus, so that there is no chance that your son and this kid end up near each other. Good luck and hope this helps!
C.