Help needed with 5 1/2 month old kitten.

Got a British Shorthair Kitten 2 months ago. He had diarrhea when he arrived , put him on RC gastro-intestinal and it went away. Now he's on Granatapet wet canned food. He's now 5 1/2 months old and 4.5 kg. The second thing that bothered me when he arrived was that after a little session of active play time he would start panting, which he still does to this day after playing (he is over 40-60 breaths per minute while playing). His breathing has become worse lately. you can hear him breathing, but not all the time, and although I'm no doctor, I think he is breathing in the abdomen. He had some dark brownish nasal discharge and some eye discharge that started brown-ish then was white-green-ish and now it's not realy there anymore.. He's had: -two blood tests  -a series of x-rays  -an ECG  -and an FIV test (which came back negative). .  

By now he's been seen by two doctors, three including the cardiologist (who recommended the dosing of thyroid hormones -waiting on the results- and another check-up 6 months from now). The first doctor told me to wait 3 months before taking the ECG because it may come back inconclusive as the kitten is still young and growing. The second one told me it's better to do it right away, to establish a precedent, and after seeing the results wanted to start him on antibiotics for a superior respiratory tract infection. He was on the anti-biotic for a week, until a week ago, and has not improved. 

I would very much like to hear your take on all this. Thanks a lot in advance, and sorry for the sorta' bad English.

I would not be surprised if there were more than one thing going on here, and the panting and abdominal breathing may not be part of the UTI. My Persil had very similar symptoms when she was just that age, and she was treated for gastro-enteritis for weeks before I changed vet and she had an xray and was found to have a diaphragmatic hernia. It can be caused by an accident, or, as in her case, genetic. It was operable, and 10 years later she has had, and still has, a normal life. At the same time we found she had a fungal infection and a UTI, so she was in a pretty bad way ( I found her in the street so she had a bad start). I just mention it as something else to look for.

Thanks a lot for your reply. I'm going to see a different vet next week. Maybe a pair of fresh eyes will know what to look for.