When standing, grip the sleeve as near the wrist as possible - making sure there is as much tension in the opponent's sleeve as possible (ie; sleeve tight) Lean back sharply at the same time pushing hard in an upwards direction to break the grip.
Or grip his sleeve as close the wrist as possible, ensure the sleeve is tight, and push sharply to the outside while leaning slightly to the opposite side.
Or grapevine - move your arm from the inside of his arm, to the outside, then back in. There is a possibility for an armlock here, if not - try and grip his sleeve for your own grip - he will not be comfortable or have control.
Finally, you can just turn for a throw "off the grip". If they're gripping that hard & persistently, they will get thrown if they dont let go - often they let go from the "fright" of you trying a technique. Grip the sleeve of the arm on your back, your other arm grips the same side lapel & turn for a seoi-otoshi (that's my option - a variety of others could work).
If he is persistant in trying to bend you over, attack with deep tomoenage and uchiki-daoshis. His grip is a defensive grip and he is more likely to be penalized. If he isn't bending you over, take his back collar with the inside grip (of his arm).