Hi, am new on here although I do have alot of experience in the animal field....

It is far from ok for your pet to be over weight. It can cause all kinds of problems - such as heart murmour and diabetes to name but a few.
My cat was incredibly overweight when I got him from a rescue cenre and he was past the science diet stage - we had to literally starve him to make his stomach shrink. This does sound cruel but he was, believe me, absolutely enormous!
What worked for me and plenty of other cats I have worked on, is to buy frozen minced chicken from your local supermarket . You just cook about a tablespoon of it in a pan and thats it. 1 table spoon twice a day. After a week you will be amazed at the difference.
When he has nearly regained his more natural weight, you can start to increase the food intake a little at a time so that it isnt too much for his stomach. Then switch to just 1 feed a day. Also make sure he is getting plenty of excersise - when they are overweight they tend to sleep alot. try get them to play shortly after a meal and inbetween and put them outside if you are able to so they at least have a walk around. The more weight they loose, the more active they will become.
Your vet should be able to tell you the best diet for your cat and also most vets will not charge you to use their scales once a week so you can record his weight loss/gain.