Aetena HMO, specialists?

So my plan says I can go to any OBGYN but I have to have a Primary Care Physician. So that's all good and well. Well last december my OBGYN referred my hubby to an infertility Specialist. We had a regular Aetena plan then. Well he didn't go and now is wanting to but our insurance switched over to the HMO. So can he still go to this infertility specialist with an HMO?