The temperature is too high. Ideally, they should be kept in temperatures between 16 and 18 degrees Celsius. Could you place some fans over the tank to help cool the water? You could also wrap some damp towels around the outside of the tank to help cool it down a little.
It is recommended not to have gravel in an axolotl tank as there is a risk of them swallowing the gravel and a potential for their gut to become impacted by the gravel which can be fatal. It is best to have either a silica based fine sand or no substrate at all or you could have large stones but these must be bigger than the axolotls head and can trap debris under the stones making it more difficult to keep clean. Although, In a cycled tank, it would be expected that there will be a colony of good bacteria on the surfaces in the tank including the filter media and substrate and you want this. I would however remove the gravel for safety. I would recommend purchasing your own liquid test kit for ammonia, nitrite, nitrate and PH. You should be testing the water regularly (at least weekly in a cycled tank, more often if the tank is cycling) and having your own kit gives you more control and saves having to cart water to the store for testing. Often, they will tell you the water is fine when there is actually an issue. Did they give the exact readings? I recommend purchasing the API freshwater master kit. You want a liquid test kit that at minimum tests for ammonia, nitrite, nitrate and PH.
Has he been pooing normally? If not, the floating could suggest he may be constipated.