My ds had that to a v worrying stage, it was explained to me that his tendons stopped growing, the short of the long story turne dout to be that he also stopped growing in other areas.
A number of things you could consider: for my ds going gluten free eased off the tightness almost instantly (his feet/toes were so stiff that he was tripping over them). BTW, there is a logical explanation for this.
Other things that you could try is anything that works as a relaxant, eg Mg/B supplement, Mg tissue salts, epsom salts in the bath, there are also nice relaxing herbal drops by Vogel.
Recently I found out that stretching does work in an opposite way, ie the tendons will snap right back, better way is to apply pressure accross the tendon/muscle, which is what I have started doing on my ds. Also, small movements/slow exersice is better than fast and energetic.
For us, drs offered shoe inserts, but they didn't work.