Well the seat is not tested and approved for use over 40 pounds. So there is no data on it and no one is going to be able to say its OK. Its probably OK, but no one can say it will be.
I will say that if I were you, what I would do is use his Scenera RF for the baby on the plane if there is room and if not, then gate check it. At your destination, use the infant seat your mom borrows for the baby and then use the Scenera for your DS1 there.
If your mom can borrow a convertible seat for your 3 yo, thats a better solution. Then, if it were *me* I'd have DS1 just use the seat belt on the plane and use your infant seat for your baby. Again, if there's no room, then gate check the infant seat.
Just be aware that even though you are technically supposed to be able to keep the baby in a carrier through security IF the carrier does not have any metal parts, be prepared to have to take him out. You're kind of at TSA's mercy on this one.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)