I think maybe moving up maybe a couple of weights might be good, ok the punches will be harder, but he is a massive lightweight, and more muscle and a bit of body fat should impove punch resistance, also working on the neck muscles maybe?
Theres not a lot you can do. Distrubution of weight more to his legs(Which they are doing) along with moving up in weight can help, but it cant cure. He's never going to be solid. Benn could be all over the shop as a Middle, but went on to take 10 rounds from G-Man up at SMW, and looked far more solid. Theres other examples of punch resistence being imrproved with a move north that folk were mentioning on another thread not long ago.
Hell if he had an iron chin he'd be invincible an where would be the fun in watching him?? I don't think moving up in weight will help him. If a fighter has been struggling with weight, then maybe... but when someone like Limond can put him on your ass, you just better concentrate on not getting hit.