We know that Avaham Avinu possessed knowledge of mitzvos. For example, he
knew about the eventuality of matzo and must have taught Lot about it since
Lot prepared it for the malochim (angels) when they came to S'dom.
Avrohom Avinu, with his superior intellect, detected G-dliness in the world
about him. His reasoning helped him see the mitzva possibilities in every
aspect of creation.
It is entirely possible that in the Yeshiva of Shem v'Ever that similar
reflections on G-dliness were discussed. By the same token, they were in a
situation of ainom m'tzuvim - not subject to the commandments. For them
to commit to writing ideas that were not mitzvos - or at least weren't yet
mitzvos - was unlikely.
Your son's question about knowing the future can be asked about any number
of references in the Torah. For example, Moshe Rabbeinu made statements
that indicated awareness, on some level at least, that he knew he would not
lead B' nei Yisroel into Eretz Yisroel long before he was actually told by
HaShem that he would not be allowed to do so. Apparently, there is a
level of knowing something that is not a complete knowing. We find this
with Yaakov Avinu who refused to be comforted for the "loss" of Yoseph.
Since Yoseph was in fact alive, he could not be comforted since nichum can
only occur if someone is niftar . Thus, he may have been aware on some
level that "all was not what it appeared to be" but he was still held back
from complete knowledge. That may be what occured with all the Avos and
with Shem v'Ever.