How Should Birth Father refer to himself?

Hi, My kids were adopted at 10 months and 7 months old- same birth parents. They have always known they were adopted. It started out as closed adoption, with non identifying gifts and updates sent through our provincial service. Over time my husband and I decided to open the adoption fully. The kids were about 4 & 5 at the time. We travel and visit the birth parents about 1-2 times per year. First Father has always been very respectful and honouring of our positions as the children's parents. The kids are now 14 & 15. Today he phoned and asked how he should refer himself to the boys. He signs cards "Your friend". He does not want to be called "Dad", he wants it clear my husband and I are the parents. How have others navigated this? He's in that in between space of friend/ family. I'd like something that 'hits the middle' so to speak. Open adoptions are unique and changing. Your thoughts are appreciated. Thanks kindly