Since the adoption is uncontested by your son's father, you may only need a paralegal to prepare the documents. You may not need an attorney.
Oh, sweetie, I am so sorry! That's awful. To answer your question, it depends what state you're in. Some states allow parents to use objects to discipline a child, and others don't. The severity of the punishment and any physical marks like bruising, laceration, etc. is likely also to be taken into account.
If this happens all the time and you're afraid to be at home, you should contact your state department of social services (usually called Department of Children and Families, DCF or Department of Children and Family Services, DCFS. )
If it's a one-time fluke and you're just curious about what it's classified as, I'd be careful about who you tell. Most states say that teachers, social workers, and even school counselors are mandated reporters, meaning that they have to report what you've told them, document it, contact DCF, and implement an investigation. That could cause internal family friction and some unintended consequences.
Most states have rules protecting confidential statements between a child and his/her attorney, so if you contacted a lawyer about your inquiry it is more likely that he/she not only wouldn't have to report it, but would be under an ethical obligation to not report it (but each state is different, so check first).
And, last but most important, if you feel you're being abused and/or want DCF to get involved, clearly, you should seek help from an adult--a teacher, social worker, or someone else that you can trust.