Can I go from custodial to full custody if my son's mom hits my stepson in front of him?

After reading this it seems your wife is not mentally stable and her actions impacting both the child very badly. I would surely recommend you to go for full custody and try to keep away child from the mother.

I don't know the answer to your question, but I'd say it would be worthwhile to consult a custody lawyer to look at your case.

Call your lawyer and ask him or her.

Start documenting everything that your son tells you. If you think your son is in danger consult with authorities and ask for a wellness check, law enforcement would show up and ask to come into her home, they may look at the rooms of the kids or even ask them if everything is ok. Furthermore, a report is made which you can go and attain a copy for your records. Start proceedings, get a laywer if you want, most ppl in these forums just say get a lawyer because its easier and makes them sound smart, but they havent been through this before, trust me, i know you are just asking for advice. Get minors consul if possible for your child if they are under 18 years of age that way his words carry some weight in court. Just make sure your documentation is detailed, take the high road in all converstions with ex and always state "what in your childs best interest". Lawyers are a 50/50 situation, they can be an immense help or prolong the case which you end up paying for. Now, I will wait for eveyone negative responses.....