Reintroducing Child to Absent Parent

My son is 2 1/2 years old. He hadn't seen or spoke to his mother in 6 months. Although I have temporary custody she has been allowed to see him from Tuesday morning to Thursday night for the past three weeks.

Although he seems to like being with her he has become more aggressive and has regressed in some areas such as potty training and sleeping through the night. I am not sure what to do. Should I cut down the visits or what is the best way to handle this?