

I have serious doubts about this William de Bologne as a son of Eustace II of Boulogne. I find no record of him anywhere other than as the son of King Stephen of England or the grandson (through Geoffrey FitzEustace) of Eustace II. I think the later is the most likely. But, even then, I have found no definite connection to a son named Tristan. What is the source of your information?
Wikipedia doesn't list a WIlliam as one of the three known sons of Eustace II and his wife Ida:
https://en.wikipedia.org/wiki/Ida_of_Lorraine
I see that this was brought up in 2018 and nothing has been done to correct the connections.