it all depends on context - the ptur of chaburah shel mitzvah for ben sorer u'moreh

  כִּי⁠ יִהְיֶה לְאִישׁ בֵּן סוֹרֵר וּמוֹרֶה אֵינֶנּוּ שֹׁמֵעַ בְּקוֹל אָבִיו וּבְקוֹל אִמּוֹ וְיִסְּרוּ אֹתוֹ וְלֹא יִשְׁמַע אֲלֵיהֶם (22:18)

1) The Tiferes Banim (son of the Bnei Yisaschar) writes that it's what a parent does which matters more than what a parent says.  The ben sorer u'moreh ignores his parents, וְיִסְּרוּ אֹתוֹ וְלֹא יִשְׁמַע אֲלֵיהֶם, when they tell him to daven or tell him to learn.  Why?  Because his parents talk the talk but don't walk the walk. אֵינֶנּוּ שֹׁמֵעַ בְּקוֹל אָבִיו וּבְקוֹל אִמּוֹ, he doesn't hear the kol of his parents learning or the kol of their davening.  Their lecture is not reflected in their own behavior.

2) The Mishna in Sanhedrin (70) writes that a ben sorer u'moreh who eats in a chaburas mitzvah is ineligible to become chayav because this type of eating does not draw a person to become zolel v'sovei and rebel.  The gemara (70b) writes that even eating a meal of tanchumei aveilim, which is only a chiyuv derabbanan, is included in this ptur.  Rashi explains the chiddush as follows:

דאע"ג דתקנתא דרבנן בעלמא הוא דאי מרישא הוה אמינא חבורת מצוה היינו כהנים שאוכלין קדשים או פסחים

The Rogatchover asks how Rashi can possibly suggest that there is a hava amina that eating korban pesach is the pradigmatic case of chaburah shel mitzvah.  The gemara on the previous amud has a din that a person can become a ben soreh u'moreh only if he eats underdone, only partially cooked meat, like you would grab off the grill if you were in a hurry and had no time to wait:  

אמר רבינא יין חי מזיג ולא מזיג בשר חי בשיל ולא בשיל כבשר כיבא דאכלי גנבי

By the korban pesach there is an issur d'oraysa of אל תאכלו ממנו נא.  One is not allowed the eat the korban unless it is fully roasted.  Forget the ptur of chaburas mitzvah -- eating the meat of korban pesach cannot make one a ben sorer u'moreh because it is too well done!

Probably the simplest answer to the Rogatchover's question is that if meat is cooked kmaachal Ben Derusa'i, either 1/3 or 1/2 cooked, it is cooked enough to avoid the issur of  נא but rare enough to still make one eligible to become a ben sorer u'moreh (Divrei Yechezkel end of #15).  

R' Shmuel Rozovsky suggested a different yesod: the ptur of chaburas mitzvah does not depend on what is being eaten, but rather depends on the context in which the eating occurs.  If you read the Mishna in Sanhedrin carefully

אכל בחבורת מצוה אכל בעיבור החדש אכל מעשר שני בירושלים אכל נבילות וטריפות שקצים ורמשים... אכל דבר שהוא מצוה ודבר שהוא עבירה

There are two separate exclusions mentioned: אכל בחבורת מצוה and אכל דבר שהוא מצוה.  The latter case is speaking about where consuming the food is the mitzvah.  The former case, חבורת מצוה, is where the ben sorer u'moreh is eating whatever he is eating, nearly raw meat and underaged wine, not a cheftza shel mitzvah, but the chevra he is eating with are engaged in a meal that is a mitzvah, like eating korban pesach. In this case the food he is eating makes him eligible to become a ben sorer u'moreh, but the context he is eating in in precludes that possibility.

This yesod helps resolve another difficulty with Rashi. Aside from the case of eating korban pesach, Rashi gave another example of chaburah shel mitzvah: היינו כהנים שאוכלין קדשים.  Why does Rashi say *kohanim* eating kodshim?  Kodshim like a korban shelamim can be eaten by the person who brings the korban, even a yisrael; it doesn't have to be eaten only by a kohen?!  

Pshat in Rashi is that we are not speaking here about a ben sorer eating kodshim, which is an achila shel mitzvah.  The case Rashi is speaking about is where the ben sorer u'moreh goes out and steals his underdone steak of chulin but joins in with a chevra of kohanim who are eating their own kodshim steaks.  Even though the cheftza of food the ben sorer u'moreh is eating makes him eligible to be chayav, Rashi tells us that if you are eating that meal alongside those who serve in the Mikdash, kohanim who are eating kodshim l'shem mitzvah, you can't be a ben sorer u'moreh.  The context, the environment, keeps you on the straight and narrow.

The flipside of this yesod is the Ramban we discussed 10 years ago which explains that the reason why the ben sorer u'moreh is chayav is כי לא הומת בגודל חטאו, אלא לייסר בו את הרבים ושלא יהיה תקלה לאחרים.  As R' Leib Chasman explains, the din of the prat, the individual, is influenced by the tzibur, by the klal.  Even though the ben sorer's actions taken on their own are not necessarily so bad, since they can have a negative influence on the community and lead others astray, they therefore must be nipped in the bud.  Where the ben sorer finds himself in a chaburah shel mitzvah, the tables are turned and it is he who falls under the influence of the community, his environment, and therefore the danger is averted.