If your kitchen sink faucet is one of the single lever designs where you use the lever to adjust for both the hot and cold water it could be that he reassembled the faucet incorrectly (assuming that this is where your leak was and this is what he worked on), creating a loop that allows both the hot and cold water to mix together inside the faucet even when the faucet is not in use. The result of this is that the loop can cause all of your hot water to be colder and all of you cold water to be hotter depending on your particular setup. If you go to another sink and let your cold water run for a bit does it start getting warmer?? If so this could be your problem.
Another possibility is he failed to fully reopen the valves after your hot water tank or changed the setting on the mixing valve after your hot water tank. Most newer installations don't allow hot water from your water heater to go directly to your hot water faucet, it is first mixed with cold water to help balance the temperature. So, after the hot water pipe exits the water heater you may be able to follow it a short distance to the mixing valve. If this is the case turn it up so you can get the desired temperature at your faucets. If you have a mixing valve you can turn the temp up on the water heater as high as you want and it will never change the temp at the faucet because the mixing valve will just reduce it back to the same temperature by adding in the necessary amount of cold water. In this case you are just wasting energy turning up the water heater. Set he water heater back to normal and look for the mixing valve.