is it signs of trouble? 7 yr old zoning in on cute girls in public.?

My son's girlfriend has a boy who just turned 7 years old. He was diagnosed as ADHD at the age of 2. When I took him to the play area in a restaurant he became fixated on this little girl. He was following her. He came to the table and said. " I think she is so pretty, but she won't talk to me". It was my first exposure to such behavior so I talked to him about not following her around. She was beginning to give him dirty looks. I apologizes to her grandfather who promptly gathered up his group and left. The 7 year old gazed at her through the glass and watched until he could no longer see her. That same night when we arrived at another location he said, "I hope she is here so I can look at her some more". This really shocked me and I told his mother about it. She didn't seem to be concerned. As this behavior continued and he zoned in on little girls more often, I was surprised to hear his mother tell me that he had another "girlfriend" in the mall play area. To me this is suspect behavior and a warning sign went off in my head. I mean, if I had a 5 year old little girl in the play area and a 7 year old boy kept smiling at her, trying to be near her and staring at her, I wouldn't be very comfortable. Is this really normal? To me it feels more like stalking in training. Seriously, these are girls he doesn't speak to, doesn't know, but on sight he says they are a girlfriend and follows them with his eyes and talks about wanting to see her again after we leave, until the next little girl catches his eye in the next play area. Any books on the subject that you aware of?