I don't have a child with autism but I work with many in a program with autism.
For children, biting is a behaviour that they exhibit when they are trying to tell you something. Usually it's "I don't want to do what you're telling me" or they don't know what you're about to do/what is coming next.
One way to try to find out why it is happening is document when it happens.
1. What happened before he bit
2. What happened after he bit
3. What was your reaction to him biting
4. What was his reaction to your reaction
.....this can help you see if there is a pattern of when he is doing it. Ie. is it always when you're telling him to do something, does is happen with mom but not with dad, etc etc or is it sporatic. then that can help you sometimes figure out what core of action to take.
My suggestions would be to use first/then statements (if your son responds to verbal prompts, otherwise using pics is great!)..... Ex. First take off your diaper, then get in the bathtub
Giving age appropriate choices (not sure how old he is) are also helpful. But make sure both options are ones you're okay with.
For example: do you want mommy to lift you in or do you want to hold my hand and climb in the tub?
Using pictures or "scripted stories" are also great. Not sure if you call them that there but in canada that's the term.
A scripted story is basically a short story with simple pictures explaining what is going to happen in a certain situation. You can make them with real pictures you take ( sometimes this is most relatable for children) or stick drawings or simple pics off computer (some kids r fine with these too).
For example a scripted story about bathtime:
First page: "bathtime" -picture of bathtub
Next page:" take off clothes" -picture of taking off shirt or w.e
Next page: "take off diaper" -picture of diaper
Next: "mommy or daddy puts helps me into bathtub" -picture of you doing so
Next: "mommy or daddy helps me wash my body" -picture of this
Next: "then I can play" - picture of this
Next: "mommy will say 2 more minutes" -picture of clock or time etc.
Next "bathtime is finished, mommy or daddy helps me dry off"
Next: "time to get dressed" - picture of this
....basically that's the idea you can add/delete pages for what fits with your routine for him. Keeping the words short and to the point is key.
The idea would be you would go over this with him at a time separate from bathtime, when reading books or something so that you can talk about it and get him thinking about the routine. Then when it's bathtime you'd go over each page show the page and do the step.
Hopefully some of that might help some!
If you have any other questions feel free to pm me!