Birman Cat - Gross weight loss?

Tao81

My 14yr old male Birman male has always been a big built fairly heavy (not fat) cat. Since a kitten I have always fed him on I Ams adult dry cat food. A couple of months ago I switched temporarily (due to the cheaper introductory price) to the I Ams 'natural wellness' range of dry food and noticed that all of a sudden he lost an enormous amount of weight (I was quite envious at the time and began wishing they could create a human equivalent! )
I immediately discontinued giving him this food and put him back onto his normal diet expecting him to put his weight back on, but this hasn't happened! In all other ways he is his normal self and seems perfectly healthy and happy.

I intend to get him checked by a vet next week because he has had plenty of time to fatten up again and although he seems well I want to know for sure, but I was wondering if anyone else had experienced this type of thing with their cat as so many people I know seem unperturbed and appear to think that it is just his age?!

P.S. We lost his mum (the cat's) last April and he missed her terribly, but he seems to have calmed down and hasn't been calling at night as he was right up to about 4 months ago.