What is wrong with my gsd?

my gsd aged 4 has had problems with pooping fro a while, he moans like a human when pooping, he takes a good few minutes to have a pap and cant stand in one spot when doing it, he walks around while squatted to get it out, ive had a blood test done and it came back ok. hes been treated for diarrhoea and is still the same, he moans when he lies down or when he moves when lying, his breath is not the best but his teeth and gums are fine, pees OK and his temperament is fine, his brother has no problems and they are fed the same, i have tried different foods, i got advised to give him science plan hd or ph at 50 quid a pop for a 7kg bag is quite high when its not a guarantee and he needs 4 weeks worth to see if it works. any help on this please.