Answer: This would be Conclusion on this topic. ksazma posed the question, "Was Isaac Abraham’s only son, yes/no?" Answering yes or no can be really misleading.
Now, was Ishmael a different kind of "son" than Isaac? It turns out he was, in a number of ways, pay close attention b/c I am not going to go back and forth with you on this topic.
1: - Isaac was the child God promised him, while Ishmael was the child he tried to have on his own, to fulfill God's promise himself
2:- Ishmael (like Abimelech,) was born of a concubine, and not by his father's own wife
3:- As a result, it was in Isaac that Abraham's descendants were to be named, read Genesis 21:12:
12 And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called.
So, was Ishmael Abraham's son? Yes! In one sense; according to the flesh. Was Isaac Abraham's "only son"? Yes! In another sense; according to the promise. Abraham's "only son" came by his only wife, according to God's only promise, and thus, it was only in Isaac that Abraham's descendants would be named. And incidentally, this is exactly the conclusion that a number of esteemed Biblical commentators have come to, e.g.:
"thine only son Isaac; for, though Ishmael was his son, he was a son by his maid, by his concubine, and not by his wife; Isaac was his only legitimate son, his only son by his lawful wife Sarah; the only son of the promise, his only son, in whom his seed was to be called."
Good day fellows, ready 2 Timothy 2:15, enjoy your day.