How to treat constipation with tight pelvic muscles.

[naturalbaby]

Ds is 6 and has been on Movicol for over a year. He's had constipation and overflow which is now much better with Movicol as there's no blockage. He still finds it really difficult to relax, push and clear his bowels.
He is sitting correctly and seems to be pushing effectively but after several tests and visits to the Dr, it seems like his muscles are too tight and he's struggling to relax properly. He suggested biofeedback. Has anyone else been through treatment like this or anything else that could help?