My baby of 6 months has erb's palsy. The left arm's movement is about ninety degrees. Will it ever go above that?

Possibly. You need to have the baby evaluated at a center that can deal with brachial plexus injuries. The brachial plexus is a group of nerves in the shoulder region that can often get damaged during childbirth. There is a period of improvement shortly after the injury then the rate of improvement slows. This is an issue for a pediatric hospital with the resources to evaluate what we are dealing with.
Depends. Spontaneous recovery after 6 months of age is limited. If your infant's recovery halted at current level for a while, surgery would be next option to consider. Consult your baby's pediatrician for further details.