Smelly cat rumbling tummy

Hi for nearly a year now my 16yr male has had loose poo. Not watery it's the pastey light brown type. Had full bloods done twice in last yr. came back renal so he on fortekor. His stomach is so noisy. You can hear it like liquids running about in there. He stinks too. He leaves the bad smell everywhere he goes it's driving me mad. His fur also smells weird. My son says like playdough! I think it's a plastic smell. Been to vets so many times he says all is fine with bloods. Gave him worming tablets which hasn't made any difference. I kept asking what else to do? I'm sure he has something wrong with his stomach or bowels. He said I could get an ultrasound done but it's so expensive and I don't even know if it will show anything! The bad smell is really getting me down it's all over my house where he sits. I'm constantly washing covers and pillows. Any suggestions?
Many thanks

Have you tried switching foods to see if that's the issue? Do you feed fish based foods?

My first guess was going to be his food is not agreeing with him.

Then read about the plastic smell, that sounds like giardia to me.

It has a sort of plasticy almost mothball smell to it.

Fecals can easily miss this too, as the cysts are only shed every so often.

Thanks guys. I did change his food as I first thought ibs. It didn't make much difference and he wouldn't eat it. I've stopped changing his food and kept him on the senior felix as he will eat it and his weight is dropping so need him to eat. He is on renal tablets and had worming drops from the vet as he couldn't see anything else to try. He also has flea drops that contain a wormer. The poo issue has been going on for a year. It's not water just loose so I don't really class it as diarrhoea. The plastic smell I find is more his neck area. It's his fur or skin. The dirty smell is his behind end which the vet just said is because he has diarrhoea. He gave us some paste to bind his poo but it's impossible to get him to take it. Even 2 of us trying to squeeze in his mouth is near impossible! I've read the gairdia info and not sure if poo descriptions match. We live in uk and I have 2 other cats one is 18 the other 1. Both healthy. His poo has been tested and was all ok. Any other things I could ask vet or suggest? Many thanks

Most definitely would start giving him a spoon of pure pumpkin into his canned food.

It works quite good at firming up the soft poops.

Not pumpkin pie filling but pure.

Thanks for all your help and advice. I saw a different vet last week who totally agreed with me that he isn't well. He tested for the giardia and another bug that wasn't in the initial test we had done. All came back negative. He said we may never get a diagnosis even if we had scans done. He believes best way forward is to put him on steroids and try to get him to have sensitive food. It could be ibs or even a tumour in the stomach either way steroids would be the option so made sense to give it a go. Going back in 4 weeks to see what the next steps will be. 5 days on from steroid injection and a change to sensitive food his poo looks a shade darker and is getting thicker. I really hope we are on to something now as it's been nearly a year!

Have you tried bathing him for the smell?