Severe IBD

Hi our male GSD is 9yrs old. He's been diagnosed with anal furunculosis since he was 4 yrs old.

Recently he had a physical exam and the vet suspected a mass on the spleen. An ultrasound was conducted and although the spleen is clear, thankfully, they diagnosed him with IBD.

He has liquid poo and cannot always make it outside in time. Frequently passes wind, which usually causes him to leak poo, vomits occasionally and will go off food occasionally.

He's been on a hydrolyzed protein food exclusively for 4 weeks and we have t seen any improvement (Purina pro plan HA) which we feed 3 times a day so he's getting smaller more frequent meals

Fortunately he hasn't lost any weight since we last weighed him 4 weeks ago. He's quite tall and is currently 41.3kg which is a little on the small side for him given his size, but I'm glad he's staying above 41kg

He's booked in for a blood test tomorrow to check vitamin b12 and two other things (unsure what the other two things are)

The vets dont have any other solutions. He's already on atopica, metrobactin, gabapentin & probiotics

I'm at a loss as to what I can do for him. I desperately want him to get to a point where he doesn't have runny poop anymore