Reading through this I might have got the wrong idea, but:

If he is eating NatureDiet which is complete, plus having a mixer which is a bulker, as someone said he is getting a lot of grain which could be making him windy.

If he wont eat the wet without the dry and visa versa, have you thought about adding a complete biscuit to balance out the grain content. Such as adding wainrights salmon/potato dry food to some nature diet?

Otherwise have you tried him on something simple like chicken and rice and then gradually introducing pre-made food back in.

Also try to feed meats better on sensitive tummys, like white meats and chicken. Lamb and beef can cause upset tumys in sensitive dogs.