My 2 yr son has lissencephaly. Are there any new additional therapies other than physical and occupational services that can help him develop?

Support group. I am sorry to learn of your son's condition. Besides OT and PT and consultation with a pediatric neurologist in case of seizure disorder, you may want to check out the support groups, such as this one: The lissencephaly launchpad:http://homepage.ntlworld.com/foliot/liss/ we may find resources that can help your son and your family.
Smooth gyra/dvlpment. How severe is his lissencephaly? Any difficulty swallowing/coordinating breathing? Any seizures? If his brain is not severely affected and he is making developmental progress with PT/OT/Speech I would intensify his services so that he is getting therapy of some sort every waking moment. Realistically what this means is that ALL caretakers work with him in ways that reinforce existing therapy.