Ok. A stepchild is considered a “child” under US immigration law as long as the marriage creating the step-relationship occurred before the child turned 18. As F2A (spouse of LPR) allows derivatives, you can choose to file one with your spouse and the child as derivative, or you can file two separate petitions. The latter is preferable in at least two situations i can think of that may or may not apply to you - if you have any concerns about the marriage (you don’t wan’t to have to start all over again from scratch with you son in the case the spouse doesn’t end up coming), or if you think you might become a citizen before the process is complete (this is because the F2A spouse petition would automatically convert to IR1, and IR1 does not allow derivatives so a separate petition would be needed for the child in that case).