Overweight blu?

I'm writing regarding my 1 yr 6 month year old male cat, we have recently started to only feed him dry food as we found every time we gave him wet food and fresh meat he would be sick, so we just feed him whiskas dry biscuits since we have we have noticed that he has put a lot of weight on we have reduced the amount of biscuits we give him, is there a min or max weight of what cats should weigh? Also any ideas how to help him lose a few pounds? Many thanks Kelly