I just got my ring sling today and noticed my son is always leaning. He's 7 months so I know he can stay up but we can't manage either outward facing (tailor sitting) or a back carry because he leans. I have it pretty tight but I don't know. On my back he's only out from his knees down and neck up so he's not falling out but its not comfortable or reassuring to have a baby who is trying his best to stare at the floor!!
Baby leaning in Ring Sling
On your back, are you sure you are getting a good seat? If his knees are above his bottom like they should be, then it would be harder for him to lean. 7 months is a little old IME for the tailor sitting front carry. Try a hip carry instead or at least a slightly off-center tummy-to-tummy hold.
Also, when tightening, make sure that you are lifting with your free hand as you tighten and pulling the rails separately to get the best fit. It is helpful for me to remember that the sling is mostly holding the baby to your body, not holding the baby itself. When I first started with ring slings, I had a hard time because I was trying to kind of suspend the baby, and that doesn't really work ;)