Can anyone advise me about my cat's diet?

[TranquilityofSolitude]

I have posted about my cat a few times and had great advice. We are having a few problems with his diet and I'm not quite sure what's going wrong.

Background: he turned up in our garden in 2012, incredibly thin and clearly struggling. We fed him in the garden for 8 months before we were able to catch him, whereupon we had him neutered, vaccinated and chipped. The vet estimated that he was about 4.

When we first caught him he weighed less than 2kg but within 6 months he had grown to an absolutely huge cat at 8kg. He had no ability to regulate his food, presumably having been extremely hungry for a long time. At this point he was eating 8 Whiskas pouches each day. We trimmed this back to 4 pouches each day and he settled at about 6.5kg. We kept feeding him 4 pouches a day for some time but he is quite fussy about them, usually eating all of the jelly but only some of the meat.

I tried to move him towards a dry food diet because the wet food was mostly going in the bin. I bought Royal Canin for fussy cats, which he likes, but he doesn't seem to accept it as a substitute for wet food, which he demands regularly. We settled into a pattern of half of the recommended intake of biscuits and one pouch each day. He eats all of the jelly on the wet food but leaves the meat.

Taking him back to the vet he was weighed again and still weighs 6.5kg. This was a bit disappointing because we felt he was eating much less. The vet suggested we try Royal Canin's diet biscuits. I bought some but he was sick a couple of times so we moved him back to the original type. However, he is still sick occasionally and it's all biscuits - they come back whole which suggests he doesn't even bother chewing them but just wolfs them down.

Thank you if you've got this far! I'm not sure what to try next given that what we are doing is neither helping to lose weight nor agreeing with him. Any advice would be very welcome :)

[RubbishMantra]

These kind of bowls stop them from gobbling their food.

[TranquilityofSolitude]

Just coming back to update and to thank everyone for their advice. In the end I bought the Stimulo cat feeder suggested by Qwebec and it's working really well. He hasn't been sick since we got it and he is also eating much less. He really seems to enjoy getting the biscuits out, too.