I will try to best answer your question. I am sorry to hear that you're going through such a difficult situation.
So far from what you described it sounds like you are taking all of the correct steps. Involving CPS can be a hit or miss depending on whether you are assigned a good case worker. I would not rely upon CPS alone as they can be unpredictable, unreliable, and sometimes incompetent.
I would recommend taking pictures of your son when he leave your house and prior to going for any future visits with father. That way you can document that no bruise existed at the time he left your home and went to the fathers. If he is returned with a bruise I recommend taking him to the hospital immediately to be examined. This is one of the few ways you can try to establish in which household the abuse is occurring. Make sure no private parts are expensosed in any photographs taken.
I would also recommend requesting the court appoint a guardian ad litem. A guardian ad litem is an attorney appointed to represent your son. The guardian ad litem does not work for mother or father and only represents the best interest of the child. The Guardian ad litem can be very helpful because they can meet with your child on multiple occasions to build a rapport. They can also do home inspections, speak with any of the parties, teachers, doctors, counselors, relatives, or any other person who may have relevant information. The Guardian ad litem also has the special ability of being able to report what your child tells him or her without it being considered hearsay. This helps gives your child a voice in the court proceedings. It also is extremely helpful to the judge in trying to weed through these he said she said situations. The cost for the guardian ad litem is generally split between the parties, sometimes the court will appoint the guardian ad litem and place the cost on the state.
I would also recommend firing your present attorney and acquiring a new attorney if you do not have full confidence in her abilities or service. I know that may be an additional immediate expense of switching attorneys but I assure you in the long run it will be cheaper and you will be more satisfied. I take over many cases from dissatisfied clients and it often ends up being more expensive having to fix what was not done correct at the outset. Sometimes the damage done cannot be fixed.
I hope that this information is helpful and I wish you the best of luck