In the second son theme, the second son receives the inheritance that the first son loses. Isaac is literally a second son, and he is a second son by way of riddle. Isaac tells us something about Christ as the second Adam that we did not yet see in Abraham. Isaac is the third ‘second’ in Matthews list of records.
Isaac the literal second son
Ishmael was the first born child of Abram. He was born before Abram became Abraham. A change of name does not make him literally, not a child of Abraham, but Isaac is called the only begotten son of Abraham.  Literally, Isaac remained the second son of Abraham. In the role of the second son, he obtained the inheritance that Ishmael literally lost. 
Isaac the riddle second son
The keys to teaching (the kingdom of heaven) are the symbols of the cross. For Isaac, the obvious one is when he is placed on the altar as the sacrifice.  Prior to that moment he represents the first son who dies desolate. The fact that he didn’t die, is a symbol of his resurrection. After that moment he is he second son who is fruitful  and receives the inheritance.
Next: Second son – Jacob
- Heb 11:17 By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten [son],
- Ge 17:21 But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year.
- Ge 22:9 And they came to the place which God had told him of; and Abraham built an altar there, and laid the wood in order, and bound Isaac his son, and laid him on the altar upon the wood.
- Ge 25:26 And after that came his brother out, and his hand took hold on Esau’s heel; and his name was called Jacob: and Isaac [was] threescore years old when she bare them.