Car seat position

My son is 1.5 and I’m about fed up with his carseat! We have a safety first 3 in 1. Trying to get it to not be tippy when installed with the seat belt is nearly impossible and we’re fixing it almost daily 🙄 He is currently facing backwards and he is placed in the middle seat. I have LATCH on both back seats just not the middle. My goal was to keep him in the middle until his sister is born in April but I’m thinking it’s safer to move him to one of the outer seats and using LATCH. Do any of you have your kiddos on the outer seats and if so what side? I did read my owners manual for my car (2007 Ford Edge) and it doesn’t say where is the best place is for a car seat besides the back seat.