15 yr old male looking sad

Hi I'm new here! I've got 3 cats. It's my boy frankie I'm here for. He is 15 yrs old and neutered. He doesn't have a litter tray he always been outside. 8 or 9 months ago he started to occasionally do a poo on my front room carpet or on the rug in the bathroom. Always at night and always the same spot. We had the carpets cleaned and put boxes or things over the spot. The bathroom rug was removed. If we shut the door it solved the problem but a few months ago he did one again on bathroom tiles and it had blood in it so we went to the vets. He said not to worry about the blood but as it was a bit runny he had some antibiotics. He said his weight was borderline ok slightly under so did a well cat check up which involved blood tests and everything came back clear. I even had his poo tested. The meds seemed to do the truck but about 5 weeks later he started again not all the time but if door left open he would do a poo at night. I could smell it from my bedroom in my sleep! Not water but very soft and patè like! He has senior food and we keep him on the same one now a good brand the felix senior. He also has royal canine senior consult to get his weight up a bit. He weighs 3.9 kg and is a long haired domestic. we saw a 2nd vet. She said she also thought it was behavioural. I suggested it could be anal gland infection and she said possibly so we had more antibiotics. This caused him to rub his bottom along the floor after going obviously where she had squeezed them. He was ok after a week or so. she also said he needed some teeth out so we thought that was possibly one of his problems and he would be happy after that. He had 3 out and recovered well. He eating ok but likes to spend a lot of time outside now. He comes in of an eve to sit with us. He just looks so sad and his weigh isn't going up. The other day I caught him doing a poo on the lawn in the grass which isn't what he usually does. Still light brown in colour and soft. Then last night I woke as I heard him doing his little miaow which he hardly ever miaows. He ran downstairs but even after I opened front and back doors ( he also has cat flap) he just wanted food. So I fed and went back to bed. 2 mins later he was back and I could hear him pawing at a cat toy which has a plastic base. He had his bottom position to poop on it so I shooed him out ANC he went straight into bathroom and sat in the shower tray. He straight away went to do a poo. It was a splat of air and wet ( only a little) and a soft poo again! He straight away ran as he knew he had done wrong so I went down stairs and put him outside. I just don't know what to do. He had the chance to go out only minutes before. This is 3am in the morn! He has had tests so I don't know what else to ask vet to do. I really don't think it's behavioural. We lost another of our cats in November he was almost 17 but had lymphoma and kidney problems. Franks poo issues started before he passed as we never knew who had done it! A few months ago we also got a new kitten (5 months old) they aren't best friends but they tolerate each other. They aren't together much and I can't say it's done to him as it all started way before. Any suggestions of what it could be and what to do next? Many thanks gemma

I am guessing from some of the terms you are using that you are European so I am not sure about the terms I use.  In the U.S. we have a test called a PCR fecal test that checks for all possible issues with parasites in the digestive tract.  It's much more thorough than the float test.  

Was his thyroid checked when blood work was done?  Being more vocal at night, eating a lot and still losing weight, and his age all make hyperthyroidism a possibility.

Yes I'm in England sorry wasn't sure where you guys were. My vets do a special health check up for the senior cat. He said it was cheaper than doing separate blood tests. So it covered kidneys and all the usual things. They have made it sound like there is nothing left to test. I asked for his poo to be checked as that was the issue! He has advocate flea drop if you have those? They are for fleas and worms. The poo sample came back normal. Can you think of any other things to text for? I have been weighing him over the last 6 months he hasn't lost weight he staying at 3.9 kgs. His fur hides it. You can feel his ribs. I know he getting old but I feel there is a problem but I don't know what so can't ask the vet specifically for anything. He just has a sad face. My girl Charlie is 17 yrs and she is full of beans and happy!

You could try treating him as if he has IBD or IBS.  Try a food with a protein he has not eaten before.  Also a member in Germany has posted about a supplement  called entero-chronic, I personally don't know anything about it because it isn't available in the U.S.  It's supposed to be very good for IBD and is often tried before going the steroid route.

Thanks I will mention all to the vet. I've book another appt for Wednesday morning. Luckily I have pet insurance!