My daughter was a tip toe walker. My husband was constantly worried about it and I laughed it off saying it was just the way she liked to walk. I walked on my knees for several months, and turned out fine, so I figured it was nothing.
About six months into the tip toe walking (I should say into the time when my husband expressed most of his concern about it continuing, since she always walked on her tip toes), my daughter started limping. We took her to the pediatrician and he said she probably sprained her ankle. It continued to get worse to the point where she wouldn't walk at all. The pediatrician said she was probably just overusing the ankle that wasn't sprained. Whenever she was home, she wouldn't walk, but at the doctor's office, she'd get up and limp, so he thought she was fine. He told us to just keep an eye on it.
Two weeks later, my daughter started running a 104 temperature and became completely rigid. She didn't want to be held or touched. She also started vomiting. We took her to the ER and they quarantined her for four days (thinking she had some strange virus) and ran all sorts of tests. They concluded it might be juvenile arthritis. My daughter was 22 months old.
We were referred to Scottish Rite Hospital and they confirmed it was JA. They had my daughter crawl on the floor and pointed out that her left wrist had lost all range of motion - something we never noticed because she's right-handed and had been compensating with her right wrist to carry stuff, etc. We also had no idea that kids got arthritis. Also, I had thought her knees looked very fat, but the pediatrician said it was baby fat and would eventually slim down. Nope - it was the inflammation from the arthritis attacking her joints.
Juvenile Arthritis is pretty rare, but it does seem to emerge around 2-3 years old and again around 10-13. It affects more girls than boys, but boys do get it. It also doesn't have to present itself like my daughter's case. Every case seems to be different. My daughter's arthritis seems to be triggered by stomach viruses. Her body tries to attack the stomach virus and then goes overboard and starts attacking itself, which presents as inflammation in her joints.
While the chances of your son having JA are slim, I write all this to say that you should have the tip toe walking checked out and keep an eye on it just in case it is more than just a preferred way of walking for now. I wish we had pursued my daughter's tip toeing sooner to avoid the damage she has to some of her joints.
Also, most pediatricians won't know anything about JA, so unless there is an extreme case (like my daughter's ER trip), most kids don't get a real diagnosis for years. So if you take your son to the pediatrician and he/she thinks it's nothing, if you notice any signs of JA, you should press your pediatrician for further testing.
JA usually presents itself in larger joints - knees, ankles, wrists, but can also show up in fingers, toes, etc. If you notice your son has limited range of motion with his wrists, knees, ankles, fingers, toes, or his joints seem hot to the touch or swollen (this isn't always constant - it can come and go), ask to be referred to a pediatric rheumatologist just to get it ruled out. There are only a couple in Dallas and it can take months to get in to see one.
Hopefully it's nothing to worry about with your son, but I wanted to share my experience with the tip toe walking as a parent who thought it was nothing to worry about and now regrets not pursuing it further.