I walk on my tiptoes.The best thing to do is to get them to quit because if you leave it like it is they'll be walking like that forever trust me I am 17 and I still do although i started after my feet were burned.
Can your son walk normal? My kids walk on their tip toes alot and my daughter OMG she curls her toes up and walks on them and me and my mother always did the same. It just depends on your child.
Did you have this seen to yet? My son was the same and has recently had surgery to correct the problem (achilles tendon's were too short as mentioned) The earlier you have this seen to the better, the older your son gets the more likely he'll need surgery. I never had my son seen to until he was 7 (nearly 8) you can read my site if you'd like.
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my cousin is the same exact way. they told my aunt that his achillies tendon is too tight and he has to go to like physical therapy to try and loosen it.