Which Simchah did he praise?
Torah Temimah citing Shabbos 30b: This is Simchah of a Mitzvah.
Rashi: He rejoices in his portion and engages in 'straight laws that gladden the heart' (Tehilim 19:9). He is not avid to increase money via Ribis and theft. Anyone who is not happy with his portion, he avidly seeks money. He comes to theft, Ribis and Ona'ah. One who is not happy with his portion regarding love of his wife, he avidly pursues women to think about married women.
R. Avigdor: The Gematriya of "v'Shibachti Ani Es ha'Simchah" equals that of 'Zehu La'amol b'Torah u'Tzedakah u'Ma'asim Tovim 1 .'
Seforno: It is rejoicing in his portion when he attains, from temporary matters, the essentials that suffice for him.
This is according to minor corrections brought in the Magihah in Toras Chayim.
Above (2:2) it says that that Simchah is worthless. Here it says "v'Shibachti Ani Es ha'Simchah"!
Refer to 2:2:6:1-3 and the note there.
What should he eat and drink?
Rashi: From what Hashem graced him with it.
Rashi citing Koheles Rabah: Every eating and drinking in this Sefer refers to Torah - "Lechu Shivru ve'Echolu
How does it accompany him?
Rav Sadya Gaon: It will be for him a companion from all his toil in his lifetime.
Rashi: It joins with him 1 - "v'Halach Lefanecha Tzidkecha" (Yeshayah 58:8). Ri Kara - the rest of his toil, others will consume it.
Ibn Ezra: His Simchah and pleasure accompany him.
Seforno: Via this he attains the intended perfection in investigation and deed, and does not waste his time to attain excesses.
Metzudas David: Due to the Simchah, when he toils, he is not pained.
Rashbam: This is like "Yilaveh Ishi Elai" (Bereishis 29:34). Metzudas Tziyon - it is like "Gam Ashur Nilvah Imam" (Tehilim 83:9).
To what does "Yemei Chayav" refer?
Rashi: The verse should be re-arranged. The only good for man in the days of his life that Hashem gave to him, is to eat, drink and rejoice, and it will accompany him in his toil.
