Is the velcro getting undone, or is the elastic allowing the fly mask to come off?
My geldings pull each others fly masks off when they are elastic--when there is just velcro they tend to stay on longer.
I bought a Cashel fly mask for my little arab cross, and his fly mask stays on for the most part. I guess it is harder to slip it over his head. Another thing I like about the Cashel is that the velcro strap lies flat against the mask, making it very difficult for a horse to get a hold of it to pull it off.
If worse came to worse, I would just put some of the roll-on fly repellent on his face. The other horse can't rip that off.