Many children when they are learning to walk , use their tip toes as it helps in their balance. In some children this goes on till 6to 7 years of age after which it resolves on its own . In a few children this may lead to tightening of calf muscles and shortening of achilles tendon which will make heel walking difficult and may require physiotherapy. Only in very small number, toe walking is due to neurological and developmantal issue.So relax , if he is doing it intermittently , he may outgrow it.

Toe walking at this age is abnormal. You need to visit paediatrician and do proper follow up urgently