My son is taking anti rabies injection and last injection is on 28-3-15. is there any problem on sending him to an amusement park on 30-3-15?

Post exposure: Is this the post exposure vaccine for rabies? If it is the same vaccine used in the US, then it is a killed virus, so he would not be contagious. Common side effects can include pain at injection site, general muscle/joint aches, and rarely fever. If he is feeling well enough, then ok to send him. Consider nsaids to help with mild side effects.

No reason not to go: Rabies vaccination should not stop your son from going to an amusement park. Did he get bit by a rabid animal? Generally the vaccinations are precautions - if the animal can be tested and test negative he wouldn't need the vaccination.