It's brought down in Halacha that if one makes a siyum during the 9 days they may eat meat. A person happens to have a siyum coming out now in the 9 days (he is only one daf away.) However not every siyum he eats meat. Some siyumim he makes by himself without a "celebration" and an official seudah, while other siyumim he makes on Shabbos (specially for the reason that he is anyway eating meat then and having a seudah that it would be nice to use to celebrate the siyum instead of doing it special during the week when he may not make such a big seudah with meat.) In short this person sometimes does make a seudah with meat for his siyum and sometimes not. May he make the siyum now during the 9 days and serve a meat seudah to his family?
This Kof-K article says
When one finishes a mesechta, there is a mitzvah to be joyous over the occasion. The simcha is so great that some poskim say one should mention simcha b’meono at the siyum. (The minhag is not to say it). The Yam Shel Shlomo says there is no greater simcha or mitzvah that is done before Hashem than the simcha and mitzvah of finishing a portion of Torah. The poskim say since there is such great simcha at a siyum, even if people did not finish the mesechta they should still partake in the siyum.
So, IF you are finishing the masechta, there is a mitzvah to make a siyum. If you make a siyum, there is a mitzvah of simcha, and that simcha is enhanced via eating meat. (En SImcha Ela Basar - don't recall, offhand the source of this, but will try to edit later.) Furthermore, the above quote states that even those who did not complete the masechta can join in the siyum and eat meat. In short, there is nothing implied here regarding what your previous custom or habit was.
Despite this, though, the same article, later on, says:
If one was not planning on making a siyum during the nine days, he should not hasten the learning in order to finish it during this time. Some are lenient with this. However, one should act in accordance with the first opinion mentioned.
Your question implies that he was planning a siyum, anyway. It's just a question of can he serve meat?
Of course it is a seudas mitzva. He is allowed to eat meat there. If he does not, it is like not eating at the seudas mitzva after a bris.