I ruptured my central slip extensor a few years back and had a similar issue with lack of flexion post-op, due to scar tissue build up.
The physios created a gadget for me out of a wrist splint with finger-glove inside and a piece of velcro attached to the top of the finger and opposing strip along the inside of the wrist splint. I had to pull the velcro down and attach it to the wrist to force flexion, repeatedly pulling it a little further every few minutes. Doing this for about 20-30 mins, 3/4 times a day worked a treat and I now have only about 5 degrees less flexion than before the injury.
Physios i've seen since have been surprised and impressed by the amount of flexion I regained after such an op, so might be worth mentioning to yours.
That was also complimented by a fortnightly ultrasound session and massage/mauling by the physios themselves. The surgeon had also suggested that if it wasn't making progress he would knock me out again and forcibly break it up himself!