Unless it's coming from the original owner of the seat who is a trusted friend or family member, knows the whole history of the seat, and is "up" on correct treatment of car seats (has never improperly cleaned it, checked it on an airplane, etc.) and whom you trust with your child's life, used car seats are not generally recommended. (If you trust the techs and advocates here, you may find one of them has one to sell if you post in our ISO forum- though you may also find that they don't since pre-2012 compliant seats aren't supposed to be passed on- and I still recommend making sure that you feel you "know" that person, because buying is always at your own risk.)
I have a daughter with SPD as well and while it took her a while to be able to go full-time, by 7 1/2, only a little later than the typical spread (5 1/2-7 is fairly typical, with outliers on both sides) she was ready for a full-time booster and hasn't rode harnessed since, and is now 9. She does still use a high-backed booster, both because she's petite and it helps it fit better, and also for comfort. You may find when he DOES outgrow the harness, that you need to try several boosters to find one that he's comfortable in.
I have a daughter with SPD as well and while it took her a while to be able to go full-time, by 7 1/2, only a little later than the typical spread (5 1/2-7 is fairly typical, with outliers on both sides) she was ready for a full-time booster and hasn't rode harnessed since, and is now 9. She does still use a high-backed booster, both because she's petite and it helps it fit better, and also for comfort. You may find when he DOES outgrow the harness, that you need to try several boosters to find one that he's comfortable in.