Loss of power

Took my son’s rzrs to local mechanic to have coolant changed and after we got it home he took it for a test ride and now it totally bogs down. He states that even giving it full throttle it will only go about 15mph. What could the mechanic possibly have done wrong? It seemed like a simple job to screw up so badly and I definitely don’t want to return to him to fix it. Anyone have any ideas?