To answer the question of who is the mother of TD's baby, I think the closest thing he has to a mum is the egg donor.
The child has a mother. She is the woman who gestated and birthed him, not the woman who donated her gametes.
Do not make the mistake of reducing the female reproductive role to the male one (gamete supply). Although this of course is what the surrogacy industry is trying to do, which is why they typically split the female reproductive role between two women.
In her brilliant book on prostitution and surrogacy, Katja Ekis Ekman notes that in the early days of surrogacy, the pregnant women were referred to as 'surrogate mothers'. As the practice expanded, the word 'mother' was quietly dropped. It needed to be, in order to disconnect pregnancy and birth from motherhood for the commissioning parents' comfort.
BTW Ekman makes a good case against ALL surrogacy in her book. She argues that 'altruistic' surrogacy should also be banned because: a) it is so uncommon (almost no women want to do it) that its main function is to provide a cover for the establishment of commercial surrogacy; b) surrogacy is intrinsically unethical, regardless of whether money changes hands. There are some things you should not ask another human being to do: going through pregnancy and birth, and then handing over the resulting child with no right to contact with him or her, is one of those things; and c) altruistic surrogacy functionalises motherhood, making it into a service that a woman performs for others.
www.feministcurrent.com/2014/10/06/surrogacy-is-child-trafficking-an-interview-with-kajsa-ekis-ekman/
As I noted above, we've seen also how surrogacy degrades and erases the female reproductive role, reducing it to that of the male, so that people seriously believe that a woman who has gestated and birthed a child is not the 'biological mother' of that child or any kind of mother at all. Even though pregnancy is a biological process - indeed the most complex and involved biological process on earth, and as mammals we evolved to depend on the maternal body before and after birth. In this insane patriarchal schema, pregnancy and birth is irrelevant to motherhood - all that really 'counts' in the creation of a child is gamete supply. It's the Ancient Greeks' "woman-as-merely-the-soil-in-which-the-seed grows" all over again.