The bath idea will work. keep the water very shallow. He may panic if he's not used to it. Be positive the drain is firmly plugged and if he's small, that he can't get into the overflow if you have one.

The key here is tepid: Your body is at 98F or whatever, "warm" to you may be "hot" to your ball python. If in doubt, go cooler. Then just gently pull a rag down his body a few times.

Your BP may even like it, as my goofy female does.