He might have nicked the flash flex or wireless charging coil flex. If one of those is nicked by a knife slightly it will cause the phone to boot loop. Back glass replacement is tedious and needs people to be extra careful.
Unsure why he would be going through the front of your phone for a back glass replacement and replacing a screw he forgot to put in. Unless you got a back housing replacement done?
Something needs some investigating as it should not be boot looping after a housing or back glass replacement.