Really important question!?

If he rebuilt the transmission then any shavings that were in the tranny would be out already. Sounds like he didn't do the job right. Check your state laws. In Ohio any work done HAS to come with a one year warranty. And for the record a cooler will not help it if it is slipping. Even with you saying he re-built the soft parts the rest should have been checked and cleaned. As for the bumper.... if the damage was not there before you took it to him then he is responsible for the damages.