He pays on time and the electricity bill will go down. . . Two big positives in your favor.
Also, he fixed something w/o bothering you - saving you time and effort.
If there is a problem later on, the lease should cover your butt. You might want to remind him to keep the communication channels open and that the lease says to call you first.
The only problem would be that if he damages the property on the install or buys a very expensive AC and tries to stick it to you. Sounds like your lease has you covered. Some renters do actually treat the place like it is there home (as opposed to those that don't and trash it). I'd rather the first type.