You are right to ask these questions. This child needs a home. You need to know what to expect. The medications he is taking indicate to me that he is a handful.
An important question to ask is how someone determined he is bipolar. What went into the analysis? Are the medications being used to treat a mental illness or to control his behavior? If he is truly bipolar, he may be on the correct medications. But antipsychotic medications are often used because they are effective in calming someone down when aggressive. The distinction is important because any medication used long term is going to have an impact on his developing brain. As far as I know, prescribing Risperdal to a kid is an off-label use. You will want to ask about potential long term side effects.
If the problem is primarily behavioral due to neglect, lack of socialization and lower than average intelligence, medication will calm him but won’t help him learn. Please, please don’t write off behavior therapy. It is often effective as a teaching tool with people who are as intellectually disabled as this little boy. He can learn. It will take time and patience but he can learn. I suggest you ask for a consultation with an ABA (Applied Behavior Analysis) certified specialist to talk this over.
A powerful intervention for someone like this child is love. He has every reason to not trust people and to be defensive and offensive in his behavior. It will take lots of time and attention to win his trust so he can look to you for protection and comfort.
In your state, the Department of Aging and Disability Services (DADS) oversees care of people like this boy. Before agreeing to adopt him, request a consultation with the mental health worker who is most familiar with his case. Make sure you are fully aware of services and financial support that are available.
This little boy will require a lifetime of care. It is an enormous commitment to take him into your family and into your heart. Such relationships can be very, very rewarding. But it is certainly important that you go into it with your eyes open. Including him in the family will effect every member. Do think carefully about whether you and your partner have the emotional energy, time and resources that will be necessary for many, many years ahead.
I wish you well.