Flat feet are a trait of DMD. My son has them, but they have never bothered him and we have not done inserts. I have heard of it though...
I am not much help! Sorry!
My son is very mobile. He is 9.5 and can still run and jump. Pretty amazing really. We are very thankful!!! His flat feet have never bothered him at all.
My 6 year old has flat feet too. He's had it for as long as I can remember. We started with shoe inserts about the time your physio is talking about because he was so clumsy. They helped him with his stability, but age also helped that too. We pull out the sole inside the shoe and then put the insert in there. He's never complained about it. On the other hand, it's so important for the muscles, tendons and such are allowed to develop without the aid of the shoe which is why I bet the physio wants to wait. My little guy still only wears shoes for school, child care and other times when feet need to be protected. Otherwise, going barefoot is great for all kids.
I have rarely had a child care about the shoe inserts. They sit in the bottom of the shoe where you can't see them and they don't feel too different than without them. I would suggest using them. When the foot/ankle/leg is not in alignment it puts more pressure on the joints and less advantage to the muscles. Generally I don't recommend shoe inserts for children with flat feet unless its affecting their function, but for a child with DMD its better to lessen the load on the joints and muscles regardless.
My son has flat feet. When he was that age they attempted the inserts. That went over like a lead balloon. My son is now 19. He started walking on his toes at age 11. manual chair occassionally at 12, went into a powerchair at age 14. I do not regret those things that I decided aagainst or didn't force on him, I should say. He didn't do steroids, he didn't do the inserts, and he refused getting his heel cord cut. Now at 19 he is starting to have discomfort and even pain from the heelcord contractures but I have always done physical therapy stretches with him and that has really helped him. We just bumped his heelcord stretches to 3 times a day. That has allieviated some of the pain. I must tell you that stretching those muscles has helped my son so much and prevented him from having to take pain meds for a long time. Now it just once in a while that he needs to take meds. Now that we bumped up his stretches; hopefully, he won't have to use that stuff.
I was like you and wanted him to be the little burst of energy he was able to be. I knew he had plenty of confinements ahead of him in his life. He was very much a part of the decision making processes. We verbally addressed the potential outcomes of doing or not doing a treatment. I have no regrets. And us parents NEED to feel that no regret feeling. Good luck!