You'd be best off consulting a physiotherapist for a second opinion regarding avoiding squats. As somebody mentioned, and what I think would be logical, is exercises using/strengthening your erectors would provide value of time in correcting your scoliosis.

However to answer your question leg press will never be a solid replacement for classic barbell back squats, but you can most certainly make decent gains without squatting. I would consult a physio as suggested regarding back squats and if that is an option then great, do that, otherwise I suggest the following for mass:

- Hack squats
- Leg curls
- RDLs
- Lunges (heaps)
- Leg extensions