Infant car seat help???


So I have a 7 year old who is in a booster. I know the infant seat is supposed to be in the middle spot in the back. Doesn't look like my son will be able to buckle very well with that set up. Is it ever better for the infant seat to be on a side? Or just teach my son how to fiddle with the buckle before she arrives? Also I do not have latches for the middle so I would have to use the seatbelt option which I am not used to...anyone else with multiple kids have any suggestions?? Thank you