I feel for your situation.

Perhaps you can print out a BP care-sheet and give it to him. It is only a couple pages long. You can say that it is some on-the-road reading to help him with his pet over the summer.

If the father's really that terrible about how he keeps his pets, you could also place an anonymous call to your local animal welfare office. Some people respond better to hearing advice from a stranger rather than somebody they know.

Ball Python Care Sheet

Alternately, you might consider buying one of the highly recommended Ball Python ownership/care books and give it to him as a 'going away present' before he leaves for the summer. You can put a bookmark in there too, 'coincidently' placed on the 'heating and environment' chapter! His dad might be a know-nothing-know-it-all, but it is hard to ignore the instructions found in a book written by an actual expert who studies these animals.

Good luck!