toe walking may be there due to HSD (hypersensitive soles unable to tolerate the texture of the ground/floor) or flat foot or an enlarged/painful heel or something else. We give intervention for toe-walking after understanding the reasons. Sensory integration (Rope-walking, bubblewrap bursting with heels etc), pediatric physiotherapy, flat-foot correction could be some of the corrective measures. I know few people who toe-walked as kids and teens, and now walk properly. Then few still walk on toes but are doing pretty fine in their lives.
Has he been diagnosed with ASD only on this toe-walking symptom? You haven't mentioned any other issue. To diagnose a 3-year old child with ASD, I am sure the doctors observed other significant symptoms. Can you email me the assessment report at email@example.com?
Also, I personally and strongly feel that giving interventions for symptoms is much more important than diagnosis of a disorder. We limit ourselves with the ASD glasses and work only inside that sphere, or we use the child's strengths to work on his weaknesses, depends on our orientation to diagnoses.