Hi! My DS does this too and I am quite concerned.
My nephew has walked on his tip toes for quite some time (he is now 7), and we recently found out that because of it, his calf muscles have not developed properly. He therefrore had to wear casts on both legs from above the knee, all the way down, for 8 weeks. He got them off last week. Apparantly there is only a 30% chance that this will work too. Otherwise it will mean surgery.
Since hearing all this, everytime I see my son walking on his toes I tell him "Liam, Flat Feet!!" I have explained what he is doing wrong too. I am very concious about this now because apparantly walking on tip toes all the time can quite often result in the calf muscle not growing properly.

Hope that helped!!