Speech impediment - preschooler adds the h sound in front of vowels. Anyone familiar with this? How to cure?

A 4 year old preschooler has a great vocabulary, but he continually says "hear" for ear, "hask" for ask, "hegg" for egg, etc. Has anyone here had experience with this in preschoolers and how did you solve this impediment if you did? If we work enough with him, he can say it correctly, but in practice, he reverts to the wrong pronunciation. Help?