Recognizing that we don’t have a full view of the OP’s situation, I have few thoughts to offer. They may not apply to the OP but might be useful to someone else in a similar situation.
At many schools like the OP describes legacies only really have juice if they apply ED. If the OP is extremely well known to the public or is on a first name basis with the school’s VP for Development the stepson may be able to apply RD. Otherwise the boat may have sailed before Jan 1.
It tends to take less $$ to become a development admit at a top LAC than a top private university but those LACs expect students to show interest. They certainly don’t want to be anyone’s fallback plan. A LAC would typically expect such a kid to interview on campus unless they live so far away that a trip to the school would be very difficult.
Like @Thumper, I have seen many great legacy kids turned down by their parents’ schools. A classmate who had been a consistent donor and volunteer dropped off our 25 reunion committee after her daughter was denied ED. The daughter was accepted RD to the school’s rival, a school with a slightly lower acceptance rate. These were both NESCAC schools.
It’s possible the stepson knows more about these school than his stepdad thinks. Sometimes kids play the college stuff pretty close to the vest. The SS may not feel he can win in a debate with stepdad or may not want SD to feel like he’s putting his beloved college down.
Sometimes kids feel an intangible about a college that’s hard for them to put into words. Perhaps the SS feels he won’t find his tribe in a socially savvy environment and senses from what he’s seen and heard that that’s what he’d encounter at the OP’s alma mater.
Some large public U’s have priority enrollment, meaning if the SS applies and/or enrolls early he’ll have a better choice of housing or class schedule. If this is the case at the school SS wants to attend it would argue for letting him apply early. Unfortunately if he gets into his choice early it may destroy any incentive he could have to apply to the OP’s school.
I would advise the OP to ask the son to tour his school but to be prepared for a sullen kid who walks on campus and immediately declares he hates it. If that’s the case the best thing to do will be to accept the SS’s decision and support it. It’s his life. On the flip side the OP should agree to tour the SS’s choice of schools. Who knows? Either SS or SD may surprise themselves and change their mind.
A final note:
@stepdad2022, your school may be a better fit than the local RU but if it’s not are you prepared for your stepson to blame you? Scratch that. If your stepson is unhappy for any reason, whether it has to do with the school he attends or not, are you ready for him, and perhaps your wife, to blame you? If he feels boxed into attending your alma mater he may always feel like it was your decision, not his. Please recognize that successful people come out of all sorts of schools. We tend to overvalue what we know and love and undervalue what we don’t.