"Avraham returned to his 'boys'. Where was Yitzchak?
Targum Yonasan: Angels came and whisked Yitzchak off to the Yeshivah of the great Shem (and Eiver), where he studied for three years.
Da'as Zekenim: Avraham sent him away at night, due to concern for Ayin ha'Ra. Some say that Chananyah, Misha'el and Azaryah died due to Ayin ha'Ra [after their great Kidush Hashem].
Why did Avraham go to Be'er Sheva, seeing as he currently resided in Chevron?*
Rashi: He only went there temporarily, not to stay. 1
The Ramban (later, Bereishis 34:2) explains that he went there to thank Hashem for the successful outcome of the Akeidah. Perhaps he went to Be'er-Sheva because it was a special location for Tefilah (it was where the foot of Ya'akov's ladder was placed). EC. See R. Bachye, Bereishis 28:10) and Bereishis 46:1). Refer also to 21:34:2:1 & 21:34:2:2.
Why does it say that Avraham and his youths went together?
Mishnas R. Aharon 1 p.73: Avraham did not feel any change. He did not credit himself.