He is definately eating too much!
My cat is a big one (big cat generally, not fat!) and he eats 1 tin per day at most with dry food out all day for him to graze on.
It may look as though he needs feeding up so needs 2-3 tins of food but when he starts putting weight on and you try and cut him down to less, he won't be happy! Cut it down now, as long as he is eating healthily and being loved and looked after, he will put weight on.
I agree with the loose stools, a change in diet will cause this and it should settle down once his tuimmy gets used to Wiskas instead of whatever cr*p he was eating before.